Happy St. Andrew's Day.
Apparently the widescreen Rockwell-esque Google Doodle today celebrates Mark Twain's 176th birthday. 176th. Oh-kay...
His work strongly evokes a sense of place and time (a bit off the beaten track for Doctor Who, being historical America, but still possibly a good guide).
If you remember the Star Trek: The Next Generation two-parter Time's Arrow you have a fair idea how he could appear in Doctor Who, as America's Dickens makes a particularly snarky Celebrity Historical hero who will no doubt be left with the inspiration to write A Connecticut Yankee In King Arthur's Court.
You could also go crazy literal with this, and have a scruffy adolescent Sam Clemens working the Mississippi river boats when a time portal brings a bunch of chivalric knights on board. Any player who makes the connection deserves a cookie.