+1 on hacking a mouse. Get a cheap, old one (I picked up several from a guy off Craigslist for 2$ each), if they're ball mice, all the better,
Open it up, the buttons are simple mini switches soldered to the internal board. just note the connections desolder the button and solder your own switches in.
If you have another mouse attached to the system, the coin switches will also act like mouse clicks.
Most of the time, that should be OK, but if not, you can use AUTOHOTKEY and some addonscripts to independently read specific mice and specific buttons, translate them to something else (say keypresses) and discard the button click.