I think this is a great idea!
But i have some question unanswered by your post (or i just cant read):
When and how will you choose to start the tournament (and let no one join)? Will the host have a 'start tournament' button?
When will the maps be picked? You can pick maps before the 'start' of the tournament, and let those attract people. Or the maps are picked after the participants have been finalized.
I'm not so sure about mod specific tournaments. I think songs with fixed mods makes more sense. Of course, if a tournament turns out to be dt only, that's fine. But predefining the mod for an entire tournament seems pretty inflexible.
Gave it a few stars