The answer is no. Setting a work of alternate history fiction 194 years after the point of divergence is perfectly reasonable. In fact, there have been works of alternate history set millions of years after their POD. But Sobel's book is unique: instead of being a fictional narrative set in an alternate history, it's a work of non-fiction from an alternate history. Starting a work of fiction in media res is fine; starting a history book in the middle of the story is not.
So what I'll be doing is not so much a sequel as a counterpoint to the original, covering the same ground from the same starting point. This will give me the opportunity to present events from a different perspective than alt-Sobel's anti-U.S.M., pro-Kramer point of view. I'll also get the chance to fill in some of the blank spaces left by Sobel, like what happened to the Hudson's Bay Company, and how John Mason was able to come out of nowhere to get the Continentalist Party's presidential nomination in 1839. And of course, I'll be able to carry on the story past Sobel's original 1971 endpoint, into the second decade of the 21st century.
As a sample of what my reading public can expect to see in the finished book, here's a few hundred words on the origins of the Hermión family.
* * *
Although many Hispano families could trace their roots back to the conquistadors of the sixteenth century, the Hermións were not among them. Pedro Hermión's father was a Greek immigrant originally named Michael Taskasaplidis. Michael fled his native country following the disastrous Nakos Uprising, eventually reaching the Mexican port of Vera Cruz on board the S.S. Hermione in 1807. The customs clerks, finding his name both unpronounceable and unspellable, rechristened him Michael Hermione. Eight months later, when Michael volunteered for the Mexican army, he underwent a second name change, becoming Miguel Hermión. The skills Miguel had learned in the Nakos Uprising proved valuable during the long struggle to suppress the Clericalist guerrillas, and within two years he had risen to the rank of lieutenant. While stationed in Mexico City, he fell in love with Rosa Dominguez, the daughter of a prosperous mestizo shopkeeper, and the two married. 
Miguel's death at the Battle of Seven Forks in 1816 left Rosa Hermión with four young children to raise alone. The family's circumstances went from difficult to dire after the Clericalists and their Jeffersonian allies took Mexico City, and the Figueroa Purge began. Rosa realized, as many others with ties to the Morelos regime did, that the only place in Mexico beyond the reach of Figueroa's ruthless inquisitors was Jefferson. Fleeing Mexico City in April 1817, Rosa and her children reached the city of Lafayette after a harrowing four-month journey. Rosa found work as a housekeeper for Henry Vining, a prosperous lawyer, and her children were able to attend the local school along with her employer's children. 
For Pedro Hermión, Mexico City was nothing more than a dim memory from childhood; he considered himself Jeffersonian, and remained intensely loyal to his adopted homeland throughout his life.  Hermión excelled at school, and Vining was able to obtain a scholarship for him at Greene College in Jefferson City. At a time when the ancient classics of Greek and Latin literature were part of the university curriculum, Hermión's Greek heritage opened doors for him that remained closed to most of his fellow Hispanos. Hermión took full advantage of his opportunities, graduating from Greene with honors in 1831, and earning a place as a law clerk at Vining's practice. After reading the law under Vining for a year, Hermión passed the Jefferson Bar. 
Hermión was interested in public affairs, and instead of pursuing a legal career, he became involved in the local Continentalist Party, one of the few Hispanos in Lafayette to do so. With his combination of ambition and ability, along with his willingness to allow himself to be used by the Continentalist leadership as their token Hispano, Hermión was able to gain a seat on the Lafayette City Council in 1835.  Four years later, as the Libertarians under Miguel Huddleston gained control of Congress, Hermión became the only Hispano elected to the Assembly as a Continentalist. Although little known to the general public outside of Lafayette, Hermión gained a reputation in the Assembly as a persuasive legislator and a superb orator. He was not above using his race to gain favors from the Libertarian leadership that they would not have granted to an Anglo. 
Hermión was easily re-elected in the 1842 midterms, but his success was one of the few bright spots for the Continentalists, who saw their numbers in the Assembly fall from 43 seats to 41. Jefferson was still the heart of the Continentalist Party, but the state that had founded the U.S.M. was facing hard times. Jefferson cotton sales continued to suffer as the world cotton glut grew worse. The problem of cotton overproduction was exacerbated by an influx of pro-slavery fanatics from the Southern Confederation, who were fleeing the abolition of slavery under the Lloyd Bill.  At the same time, Jefferson's position as the political nexus of the U.S.M. was being undermined by California, which was growing rapidly in both wealth and population thanks to the ongoing gold rush. By the time the Jefferson Continentalists held their annual state convention in Henrytown in May 1843, the state's future seemed bleak, and the party's with it.
1. Esteban Reeves. The Hermións: Mexico's First Dynasty (Mexico City, 1977), pp 26-28.
2. Maria Fuentes Carter. Rosa Hermión: La Madre de Mexico (Mexico City, 1994), pp 61-79.
3. Herman Muller. Hermión of Jefferson: Patriot or Traitor? (Mexico City, 1969).
4. Jason Altmayer. Young Pedro Hermión (Jefferson City, 2003), pp 41-46.
5. Ibid. pp 160-74.
6. Robert Fisher. Assemblyman Hermión: The Legislative Career of Pedro Hermión (Mexico City, 1974).
7. Augustus Clayton. Flight from Freedom: The Slaveowners' Exodus of 1841 (New York, 1999), pp 276-83.