In Appendix C that accompanies _Mark Twain's Letters, Volume 5_
titled "Mark Twain's 1872 English Journals" Twain made the
notation:

"Write up Bummer & Lazarus & Emperor Norton."

Although Twain had written about the two dogs Bummer & Lazarus
during his newspaper days in San Francisco along with a few
scattered comments about Norton, according to the reference
notes for Appendix C, "Clemens is not known to have written
anything further about Bummer, Lazarus, or Emperor Norton."
(p. 625.)

At least one web site devoted to Emperor Norton does list him
as the model for the king in Huck Finn.  Unfortunately, no
citation for this theory is given.

Barb