We'd need a hard definition of "quantum supremacy" -I believe there have been several press releases claiming this already, and I think you agree with me that there are no such machines at hand.

There's this ethereum thing called Augur we could use to place the bet, though that's an interesting bet in itself (ethereum and auger being around in 20 years is not a sure thing). I suppose also "long bets." If you google my name you can find my contact info.