Gwern's SR literature review suggests that answering questions is pretty critical for learning stuff. How about a collaboratively edited math test? Every club member contributes 1 question to each test, and every member takes the entire test (except their question).

BTW, LW math study has kinda been tried in the past; I didn't follow too closely and I'm not sure how well it turned out.

I feel like a mailing list would be a good idea for accountability and coordination. Most people here probably check their email more frequently than they check LW, and a mailing list would allow you to not spam LW with stuff that most LWers didn't find relevant (although maybe they'd want occasional summary posts). Maybe you'd like to create a google group or something and add it to the body of your post?