I've also been reading a book entitled The Heads of Cerberus, by "Francis Stevens" (a pseudonym for Gertrude Bennett). It was originally published in 1919, I would guess in magazine form, but the copy I read is a small-press reprint from 1952. It's about three people from the present who are transported by some strange dust into 22nd-century Philadelphia. It's very much the sort of pulp Wolfe parodied in "The Last Thrilling Wonder Story," and it's pretty bad, so much so that after I got halfway through I just skimmed the rest. But, though there are no thematic or structural similarities I can detect between Heads and FHC, there are a few odd points of similarity which make it possible that Wolfe read
Heads and incorporated some allusions to it. These are:
1) A society frozen in stasis and ruled by a tyrannical government
2) People designated only by numbers (in Heads, everybody but the elite in future Philadelphia
3) A library which the protagonist(s) are forbidden to enter.
I can post or email a description of the book if anyone's interested.
--Adam