I would leave that to those with more varied encoder experience -- I've been using JPacs exclusively for quite a while.
What I had in mind would have been something like the KeyWiz Eco or maybe one of the lesser-known encoders, if you can pick it up for cheap on eBay (the TOKN16 comes to mind since ghosting isn't likely to be a problem if you use the mouse buttons for the player buttons). It seems that the TOKNmedia site is down, however and if you are going to pick up a keyboard encoder, it might be worthwhile to use an integrated encoder like the Mini-pac.
If you are already looking at a mouse hack, then for two additonal buttons, even I (lazy as I am) would hack a keyboard.
Depends