Lots of thanks to Qole; I was able to modify his instructions to get x2x working with all the normal keyboard buttons to share my laptop's keyboard and mouse.

I did have to make one change; enter wasn't working quite right. I had to press it twice on the terminal, and it didn't work at all in some other programs. Editing the pc file under symbols (both copies) to change KPEN to Return and RTRN to KP_Enter (to match the default rx-51 file) fixed that, though.