There was no response to this last time, so I'm re-posting it:

The user can select the "word size" hexl will use with defcustom (it 
currently screws up if you change this size behind hexl's back, ie after 
hexlifying a buffer but before unhexlifying it: this could be fixed by
saving the word size used in a a local variable in the hexl buffer, let
me know if you think this is important enough to add). Other than that, 
the hexl code doesn't seem to have changed since I last updated the patch
from emacs 22 to emacs 23.

The word-size could also be chosen at invocation time via a prefix arg, but I 
have not done this yet either: Again, let me know if you think it's
worth adding.

I originally prepared this patch for a friend who was debugging something and 
needed other word sizes: Upon investigation I discovered that the
hexl binary had support for the word sizes 8, 16, 32 and 64 bits but the
hexl mode code only supported one of those - hence this patch.