I will just search for it, but there have been tournaments that paid in Bitcoin prizes. {Not DotA2}
This should not be too complicated... Host the Bitcoin League on a Fan forum, that are already out there. Take subscription entries, place it in a pool and at the end, pay out to winners.
Have you asked this on some of the DotA2 forums yet? If YES, what was the response?
I will test the water, by challenging someone to play you for Bitcoin. Other people will ask you, what this Bitcoin thing is, and they will also want to get into the action for the money.
You can even make it easier... Ask people to go to a reddit thread you created and if they win, you tip them with ChangeTip... that will automatically start the road towards Bitcoin adoption.
You take it from there, and grow it bigger.