You could always model it after Twiddler or phone style keyboards. Multiple presses on the same button would select a different letter/number.

In the end though, I get the feeling that any hacked solution you come up with isn't going to be very satisfactory for you. At some point, you're going to have to work out a solution which either involves hacking an existing DDR pad and fussing with some relatively complex software to get the key button presses or constructing your own DDR-eque pad using something like KeyWiz(s) to capture the keystrokes and futzing less with the software side.
If it were up to me, I would op to futz less with software and more with hardware. Ensuring you get the hardware keystrokes sent to the computer in the fashion you want gives it more portability for the project. Especially considering that you're using an Apple (OS*?) and others who would want to copy your work are more than likely using a windows based PC.