Got your PM, but thought I would mention it here.
There are a number of PS/2 (Keyboard and Mouse) to XBOX converters out there. They go by the names of "SmartJoy Frag", "Max Shooter" and so on. They have different plastic cases, but appear to be nearly identical in operation.
They are programmable, so you can can set up multiple configurations in the device that match how you want your panel controls to be mapped, but you'll need to do this first with a keyboard and save it to the converter. Then you can plug in your KeyWiz and it should work fine, based on tests I have done in the past.
There are a couple of drawbacks with these types of adapters, though. You will need to "activate" your profile by pressing a button on your panel that is mapped to the F key you stored it in when you first power up (not a big deal), and you can only use one controller input on the XBOX. For a media center application or single player games, that's probably going to be fine. But it won't help for 2 players.
RandyT