Michael (if I may),

 Could Pap Finn be seen as symbolically representing the worst of
the KKK thinking?  Twain's descriptions of him, at least, capture an
image that is to evolve a little later.  I am especially thinking about
chapter five of _HF_, when pap surprises Huck with his appearance behind
long and tangled hair, "and you could see his eyes shining through like
he was behind vines."  And Twain's insistence on the whiteness of pap's
face: "it was white; not like another man's white, but a white to make a
body sick, a white to make a body's flesh crawl."
      And Colonel Sherburn's insight into the "Southern fashion" of
lynching in the dark in Chapter 22 seems to suggest at least one aspect
of the Klan's style: "and when they come they'll bring their masks."
 I know this is not the direct evidence you needed, but I just
thought Twain might be pointing indirectly at a certain part of the
Southern mentality and perhaps to particular actions.

---Jason Horn