that must be a pretty obscure bios you have, that it refuses to boot without a keyboard, and no option to disable that "feature"
check in your setup screens for an option that reads "Halt on: All Errors" ... its usually in the first screen or basic setup - there's sometimes other options too, Halt on: Keyboard only, Halt on: None ... none is what you want
Alternately you could impersonate a keyboard using a PIC or similar microcontroller. Or grab a bulk lot of nasty used keyboards off ebay, discard the keyboard itself and just keep the interface logic. Repackage the interface logic in something convenient and you're all set.