There is an article on Barnum in _The MT Encyclopedia_, but it is of
precious
little help in establishing a direct link between Twain and Barnum.  An
allusion to Barnum's house, "Iranistan," apparently occurs in the section
entitled "A Word of Explanation," under the subheading, "The Stranger's
History," in _A Connecticut Yankee in King Arthur's Court_.

Sorry I can't be of any more help.

Rich Tuerk