Something like a t9 dictionary might be useful for it, even if you'd probably need the other device to handle the dictionary unless you wanted it hacky as all hell.
Replacing the thumb button with some kind of clicky D-pad (D-pad mounted on a microswitch) would be a somewhat easy way to add more buttons. (my first thought was going with two per finger by adding one around the side or to a strap on the outside of your fingers)
