I think you've nailed it.

Clearly the author knew more then he was disclosing. It's very possible that he was presenting just enough detail to attract the attention of those with existing knowledge in his search for that "missing unintelligible piece of paper." This was still the missing piece and he clearly tells his readers this.
In 1884, just one prior to the pamphlet's publication date we have a Thomas J. Beale in Richmond, this name being consistent with the story and 1884 being consistent with the pamphlet's publication date.
TJB was said to be dark and swarthy, this being consistent with a man of color, which Thomas J. Beale of Richmond was.
Our author claimed to have important business affairs in Richmond, this being consistent with the presence of Thomas J. Beale in Richmond.
The names, Jackson and Ward are referenced in the pamphlet, Thomas J. Beale resided in Jackson Ward, Richmond.
Now there is nothing conclusive here but it certainly raises a few flags worth our investigating.