Not sure where you get the idea that such a thing wouldn't be allowed in Games and Rounds. Just have a look at the board, you'll see that the threads are divided into Bitcoin based(obviously) , Dogecoin/altcoin based that mods haven't bothered moving to the correct section, and lastly the large portion being really random stuff(services, "Interest check", questions about giveaways which technically shouldn't be made into a diff. thread etc)  that mods wouldn't dare start touching with a 10 feet pole.

As long as you specify that the game will in the end be paying in BTC it should be fine anyways. And as for the part about creating a separate board, there hasn't been a need for it.

As for keeping it 'relevant' and spam-free, make it self-moderated and bump it everyday. If people want to play they'll join even with any harsh rules