I'd independently invented the idea of a transparency one-time-pad, though I used two-pixel blocks instead of four-pixel ones. Interesting application, but I don't quite understand what it really buys. One of the requirements of a good voting system is that it be possible after-the-fact for anyone to prove how a voter voted--even the voter himself. I didn't see how the receipt could proof that a vote was counted without providing proof of how it was cast.
The voter knows how he voted, and he verified that the machine correctly recorded his vote by reading the printout before he separated the two laminated layers. Then he can verify that his vote as cast was included in the total (as described in Chaum's paper). But his receipt does not (and cannot) tell anyone else how he voted, just that his vote (whatever it was) was correctly tallied.