The article that Cindy referenced to the _British Trade Journal_, October
1, 1907, p. 377 titled "New Catalogues and Price Lists" is available on
google book search and does make reference to the Gaumont company filming
various pageants including Oxford and Mark Twain.  I think it is likely
that such a film was made.

YouTube currently includes a number of films from 1897 of Queen Victoria's
Jubilee (which Mark Twain attended) and they appear to be filmed from
various camera angles. (Picking Clemens out of the crowd in those films
would be a supreme challenge.)  The fact that footage from 1897 still
survives seems to offer hope that footage of the 1907 Oxford pageant may
also survive.  It does seem unusual that Clemens would make no reference to
the filming in his autobiography. Was the film the historical reenactment
of British history that he described in his autobiography or the awarding
of his honorary degree, or scenes from both?

However, another more pertinent question is whether or not the entry for
the film on the IMDB website (which shows the company issued hundreds of
films) was compiled from a printed catalog or whether it was compiled from
some public or private archive of actual footage.

Barb