Because of my highly limited time this 7DRL, I wasn't able to make Hellmouth happen anything like I'd hoped. Instead, I ended up releasing what amounts to a combat system test called MEAT ARENA. More information about that here:
http://7drl.org/2012/03/17/hellmouth-day-six-seven/
Despite being very minimal, MEAT ARENA is playable and winnable. I'm going to count it as a success. Hooray!