The publisher of the original book, the Macmillan Company, is now part of an international media conglomerate based in Germany. They only accept submissions from literary agents. Del Rey Books, which has its own alternate history line, is also owned by an international media conglomerate. They too only accept submissions from literary agents.
There are some SF/F genre publishers that accept unsolicited manuscripts. Tor Books requires a cover letter, three or so sample chapters totalling some 10,000 words, and a story outline, and has a turnaround time of about six months. Daw Books requires a cover letter and a complete manuscript, and has a turnaround time of three months (if you don't hear back from them by then, you're free to make a simultaneous submission to another publisher).
An additional difficulty is that FWoaN fell into a gray area between fiction and non-fiction, being a work of fiction that presented as a work of non-fiction. Thus, it may be too nonfictional for a fiction publisher, and is probably too fictional for a nonfiction publisher. Sobel was able to get away with it because he had already published half a dozen books by the time he started writing FWoaN.
Well, I suppose if I have to write some or all of my sequel before I can find a publisher, I'd better get at it. Below is another sample of what it will be like, continuing on from what I wrote yesterday. I've decided on Scorpions in a Bottle as my working title, which was Sobel's own original title for FWoaN. And so here, appropriately, is an account of the Pedro Hermión speech that produced that phrase.
* * *
The story of the Henrytown convention is a familiar one. For two days, Continentalist stalwarts such as John Berrien and Homer Brown reminisced about the glory days of the 1820s, and made vague, unconvincing predictions of their imminent return. On the third day, May 7, convention chairman Peter O’Gorman was persuaded to allow Assemblyman Hermión to give an address, largely due to the lack of any alternative speakers. O’Gorman was expecting Hermión to make a few remarks on Lafayette’s growing Hispano community. What he got was a searing critique of the Continentalist Party itself.
Hermión angered his listeners by accusing them of blindness and timidity. When the convention delegates began hurling racial slurs at him, Hermión scolded them, saying “You have just given the world another example of this blindness of which I speak.” Overriding the delegates’ howls of outrage, Hermión spoke of the party’s historic role in creating and building the U.S.M. “Our party made the U.S.M. a major world power. And now our party is in despair, not because of any failing of the party or its leadership, but because of events our party could not control: the price of cotton has fallen a few centavos.”
This was the point where previous speakers had promised that the price of cotton would rise again, and with it the fortunes of Jefferson and the Continentalists. Hermión, however, had a very different message for the delegates. He said, “Our party has not been built on cotton, but on men! And it is men, not cotton, who will bring our party to greatness once again!” And with each mention of the word men, Hermión slammed his fist on the podium, with a sound that could be heard throughout the convention hall. Here and there in the hall, the delegates who had been cursing Hermión just minutes before were shouting their approval.
The Assemblyman from Lafayette was winning over his audience, and now he was ready to bring them his vision for the future of the party of Hamilton and Jackson, and of the country they founded. “Russia looks longingly at California, Spain dreams of a new empire in the Americas. And most of all, the Confederation of North America threatens our very existence.” Hermión spoke vividly of the newly-united British dominion to the east, led by the Butcher of Michigan City and the Caesar of the Northern Confederation: “Along the Mississippi and Arkansas, in the Gulf of Mexico and the west Atlantic, we face the North Americans, who hunger for our lands and wealth.”
Hermión then concluded with the famous image that gave the speech its name, and has haunted the thoughts of two nations ever since. “In Mexico del Norte the Mexicanos have a game – some call it a sport. The peasants put two scorpions in a large bottle, and then take wagers as to which will win the struggle. Slowly, the scorpions circle each other, until one lashes out at the other, and strikes him dead.
“So it is on our continent. At first glance, it appears North America is a large place, with room enough for all. But the C.N.A. and the U.S.M. are both inhabited by aggressive and expansionist peoples. Within a few years this great expanse will seem small indeed, as we meet in the waters of the Gulf and along the Jefferson-Vandalia border.
“At this point, the scorpions will meet in combat, with only one the victor. I mean that victor to be Mexico, and I believe only the Continentalist Party, revived and restored, can lead the nation to such a destiny!”
Assemblyman Hermión then stepped down from the podium to the sound of thunderous applause. In fifteen minutes, he had won over the leadership of the Continentalist Party, and set it on a new path. 
The Scorpions in a Bottle speech, as it has become known, was controversial at the time, and has remained so ever since.  Was Hermión calling for an attack the C.N.A., or was he warning Mexico to beware of North American aggression? President Huddleston and the other leaders of the Liberty Party believed, or claimed to believe, that it was the former. When he learned of the speech, Huddleston warned that “Assemblyman Hermión should have better sense – or better scruples – than to speak so recklessly of coming war. We have troubles enough in our own land without seeking new ones abroad.” 
The Continentalists were electrified by Hermión’s speech, and he quickly became a leading figure in the party. When the Assembly reconvened in August, Minority Leader George Culpepper stepped down, and the Continentalist caucus chose Hermión to succeed him. Under his leadership, the Continentalists offered a new military appropriations bill that would increase the size of the Mexican army to 200,000 men and authorize an expanded naval construction program at the navy bases in Vera Cruz, Tampico, and Henrytown. Although Assembly Speaker Nathaniel Butler denounced the bill as “provocative and unnecessary,” Hermión was able to gain enough Libertarian votes to win first passage of the bill in September. In a speech afterwards, the new Minority Leader vowed to win final passage of the bill the following March. “On that day,” he claimed, “the people of Mexico will learn who is prepared to defend them, and who is not.”
1. The definitive account of the Henrytown convention and the Scorpions in a Bottle speech is Raúl Peterson’s The Henrytown Convention and the Birth of Mexican Nationalism (San Francisco, 1989).
2. The common view in the C.N.A. is exemplified by Janet Holt’s Demagogue and Dictator: The Life of Pedro Hermión (New York, 1954). Reeves points out that North Americans tend to view Pedro Hermión through the prism of his son Benito, who became dictator of the U.S.M. and launched several wars of aggression during his twenty year rule. Pedro Hermión himself was an elected republican leader who scrupulously maintained constitutional rule in spite of repeated invasions of Mexican territory by North American armies. Reeves. The Hermións, p 86. Mexican nationalists see Hermión’s speech as a prescient warning of Henry Gilpin’s determination to attack the U.S.M. See, for instance, Emiliano Vega Pérez. Una Sirena en la Noche: Los Escorpiónes en una Botella Discurso de Pedro Hermión (Mexico City, 1991).
3. Charles Pearson. Huddleston and Hermión: The Rivalry that Shaped Mexico (Jefferson City, 2007), p 229.