Custom PC input peripheral
I'd consider hacking an USB keyboard.

Basically take a keyboard, open it up, and wire your control panel to, say, function keys? 

For "bonus" points, you can try wiring up 'non-existent keys' (keys that would exist on a matrix keyboard but have not been populated in the most popular layouts (F13-F24's come to mind, most PC keyboards leave it at F1-F12). They probably still exist as keys on the interconnect matrix, just the key array itself is not wired for them. (Lookup Matrix Keyboard for details, or indicate in comments if you're interested in additional explanation).