Another sub-forum just for sign-ups seems kind of redundant, as most recruitment for tournaments and gyms are done on the first post anyway. It would basically be like posting the same thing in two subforums, which would be kind of confusing, and I don't know how much it would in terms of more signups. Members can clearly see all the gyms and tournaments anyway =/
The thing is, there are so many gyms and tournaments in the Battle Stadium already, so there's no way that they're all going to be full and functioning. I mean, you can't tell people not to make their own gym or start their own tournament, but they're kinda shooting themselves in the foot, as more gyms and tournaments means that it's less likely that their all going to be successful. There's only so much one person can do, and with the humungous cluster of threads there, they have to make a choice at some point. So with six or seven or however many active gyms we have right now and a limited number of people interested in them, more gyms is not necessarily a good thing.
Anyway, back on point, a sub forum just for sign-ups won't really do much I don't think. If that section were to be more heavily moderated so that random and low-quality gyms and tournaments don't get through so easily, much like what is done over in S&M, it might help some better tournaments and gyms with unique ideas get through, which could get that section a bit more activity...but meh.
Anyway, that's my take on it.