I wish we could do this but playing without real money wouldn't work for improving rationality and I don't think it would even work for testing rationality in a meaningful way.

Also, I think this sets a good precedent. People who will bet on cards are far more likely to bet on their beliefs in general, which is a sign that they actually have beliefs as opposed to just "things they would like to be true". Not taking +EV bets that represent negligible portions of your net-worth is epistemically irrational so it seems like a rationalist community should be encouraging betting when it is done at reasonable stakes as a way to train or test beliefs.