IIRC there is a bit of empty space inside the duo near the front, if you have the schematics/understand the logic of a duo tap, you should be able to build an equivalent with some prototyping board, ics, and 8-pin mini-dins that would fit in that space. the main problem I see is getting it mounted where it won't move around and still look nice, assuming you have enough room to work with. Sounds interesting but probably won't be easy.