This is why I I decided against saying "impossible" and instead "hard if not impossible". :)

I'm not 100% certain it is impossible, as there are certainly some clever possibilities that become available with zero knowledge proofs. The big issue I see is that it has to somewhat go beyond ZK proofs, as you can't have a situation whereby the voter can in a separate channel show how their vote was cast.

For example, Charlie votes for Bob in a ZK way, so Alice does not specifically know /how/ Bob was elected, only that the result of the election is true. This is a great start, but the issue is whether or not Charlie can tell Bob (off-chain) "hey, I voted for you, here's my side of the proof." Bob could then incentivize Charlie after the fact, without Alice being aware, and the issue remains.