I have Darren's MAME running on a MIPS-based HPC, so you might try recompiling. In my case, it was "just" a matter of recompiling the code and setting the appropriate keycodes and maybe the video buffer starting address.
Note that I don't have any way of knowing if there are any special difficulties with the HP Jornada units, I'm just speculating that it "should" work. Sometimes I think that half the work was fighting with the compiler (or is that learning how it works? :-) ).
> Since Jornada 680 does not run ARM, is there a version compatible with 680's