Just thinking that there are two ways to accomplish what I mentioned above-
1. Restrict member from sending PMs and from posting in GFST.
2. Allow PMs, but restrict GFST to be visible only when logged in to an account with the requisite non-GFST post count.
I think I like the second one better, as it'll also prevent people from joining DDF to try to eventually get to GFST. Other than people who have already seen it, hopefully future scammers (and innocent people) won't know that it's a benefit of a certain post count.
Not sure if either of these options are technically doable, but maybe?