She's mentioned in "Tom Sawyer's Conspiracy," in _Huck Finn and Tom
Sawyer among the Indians and Other Unfinished Stories_ (U of California
Press, 1989).  Tom Sawyer suggests various adventures to Jim and Huck,
including civil war, revolution, and insurrection, all of which are rejected
(it is
Jim who rejects civil war), until he comes to conspiracy.  Huck comments on
the civil war idea (p. 138):

"But he [Tom] give up the civil war, and it is one of the brightest things
to his
credit. And he could a had it easy enough if he had sejested it, anybody can
see it now.  And it don't seem right and fair that Harriet Beacher Stow and
all
them other second-handers gets all the credit of starting that war and you
never hear Tom Sawyer mentioned in the histories ransack then how you
will, and yet he was the first one that thought of it."

Here's a recollection of Stowe in the Nook Farm community in Hartford from
Chapters from Mark Twain's Autobiography (North American Review):

"In a diary which Mrs. Clemens kept for a little while, a great many years
ago, I find various mentions of Mrs. Harriet Beecher Stowe, who was a near
neighbor of ours in Hartford, with no fences between. And in those days she
made as much use of our grounds as of her own, in pleasant weather. Her
mind had decayed, and she was a pathetic figure. She wandered about all
the day long in the care of a muscular Irishwoman. Among the colonists of
our neighborhood the doors always stood open in pleasant weather. Mrs.
Stowe entered them at her own free will, and as she was always softly
slippered and generally full of animal spirits, she was able to deal in
surprises, and she liked to do it. She would slip up behind a person who was
deep in dreams and musings and fetch a war-whoop that would jump that
person out of his clothes. And she had other moods. Sometimes we would
hear gentle music in the drawing-room and would find her there at the piano
singing ancient and melancholy songs with infinitely touching effect."

Jim Zwick