Best choice would be a continueing posting requirement, I think. I mean people would be required to have posted once or twice during the last week to have download privilege.
This would encourage regular forum leeches to keep posting. And perhaps they might even post something else then just spam since they weren't posting it all at once.
That's a good point. Adding that to the requirement could help keep people involved -- though it would mean more policing, because it might not be as easy to set it up to be automatic. Maybe see how it goes with the 15 post requirement for starters, and see how people respond to it -- whether they spam a lot, or actually learn to participate. You can always tighten things up later if it doesn't work in the way you want it to. As for spam posts not counting towards the quota, if they are obviously spam for that purpose, they can just be deleted -- people will learn to post properly if they want their GB chapters!
I'm hopeful, though, that the rule will work well in helping folks get in the habit of posting, and then they'll be good to go. Like me -- it seems like I just joined up and here I am, almost at 100 posts!