That's the right thing to do. Since this campaign don't seem to be picky on what the participants do, then at least let them know they need to have a certain level of activity per week.
But why is it so important to you, to have requirements risen for all the other members of this campaign?
You jelly
? It was advertised as the easiest signature camp. ever and this is exactly what we are getting.
Believe me when I tell you, when people are not under post quantity pressure, they write far more better content.
I think it is one of the best approach to signature advertising, for sure better in terms of spam! Look at those sig.campaigns with minimum 20 per week... my my, some serious nonsense there
and almost nobody gives a damn.
I could agree to "at least one post per month" rule, forum is full of crappy posters already, why add more?
edit: There is one more benefit of "no post required", you know that I'm writing to you, because I want to, not because I need to.