So you pay a yearly $20, which really isn't for anything but membership.
Then you pay $15 (ded) for a new mag when your old one is sent in?
I think your legal problems only arise when the manufacture date is post 94. At first blush, i don't think it'll fly. My reasoning is that the ban didn't really do anything other than prohibit new production for the public, which wasn't a really effective ban since we can still possess, shoot and buy high cap mags. The legislative force was only in banning new production for us. That's just my take on the situation.
What would be done with the old mags? Sent to be destroyed? They would have to be, since they could turn around and be sold back the public thereby increasing the amount of mags on the market. Too many problems involved here in my opinion.
Maybe if it were only a replacement of the mag body, it might fly, but if the spring/floorplate/follower were not old parts, you just violated the 94 ban!
As far as business potential, I think you're finding a solution for a problem that doesn't exist. Now if you were able to develope a new product for the ar15, then you're in business!