I haven't really been keeping up with current technology, I kind of gave up on it all in disgust after various happenings on the Playstation forum I helped run.
I'd assume though (assumption being the mother of all @#$%-ups, but even so) that it would be far easier to build an adapter for a PSX style controller with it's multiple signal lines than it would be for a USB device. I think the hex for the PIC is bundled in the archive for the PSX hack.
Logitech also do a nice range of wireless keyboard and mouse packs which plug in via the PS2 ports, it might be easier to make a PS2 adapter and get one of those packs. They work quite well with a wintel, I'm using one on the monster at the moment. They're relatively cheap too.