From what I can tell, the game uses the accelerometers in the nunchuk and wiimote controller, with (if the 5 buttons left to right were labeled are A-E) nunchuk left for A, nunchuk down for B, nunchuk right OR wiimote left for C, wiimote down for D, and wiimote right for E.
From what I read, when not moving, all wii controller accelerometers put out about 1/2 Vs for each axis, with the voltages going up or down to indicate movement on each axis.
I figure if the buttons were set up to cut in between the chip and the board and feed in Vs or Ground (depending on direction), the buttons could simulate movement of the controllers. (I attached a rough diagram showing the circuit, although I may have the Vs and Ground connections backwards, I really don't know.)
If done correctly, the Pop'n controller could just connect to headers on the nunchuk/wiimote, and normal operation could be restored by disconnecting the cables and attaching a couple jumpers to each.
- popn-wii.png (19.46KiB)Viewed 2544 times