Rather than trying to tell everyone that they MUST tell you every block they've found, you simply need to ask people to tell you and pay them for doing it.
Those who DON'T want to tell you will ignore you. Those who can be bought easily will tell you.
Excellent! Is there a way they can prove that they found it? If so, does it require cooperation from the pool operator, or is it possible to make such a proof against the pool operator's wishes?
Slush runs two separate pools effectively, each connected to a different version bitcoin daemon. You choose to mine on one or the other pool. It is all done on a pool level.
Perfect example then. When a miner chooses the Slush pool for one version, any blocks found using their hashpower will have that version. I assume that Slush pays people in both "effective" pools from rewards earned by each pool (so everything is shared). So in order to reward a Slush pool participant for choosing one of the versions, I would have to find some way to get a bitcoin address for that participant. Kano's idea of asking is great, but can't anyone claim to be the person who mined a block? How would such a claim be verified?
Thanks again!