I don't think there will be any active DM role in it. The user-generated content would work just like the regular company-generated content, running through the script for whoever came along to play it. I think it would be analogous to one of us making an area for NWN Layonara, complete with spawns and traps and NPCs and goodies, and then having it appear in-game to be played stand-alone as-is, except that there would be lots of separate instances to cover however many hundreds or thousands of players may be trying to play it at once.
I hope that makes sense.