Sounds like a good plan, when's that leader of the TSR math soc thingy going to be elected, I'm sure they could sort something out. And hopefully it could be regulated so that it doesn't get filled with "Which uni should I go to with my grades?" or other pre-uni type questions.
The only downside I could think of is that if everyone is like me they'd go to the uni sub forum first to check threads before helping out with any A-level questions they might know. They can't complain though, they get solutions in the back of their books!