Hi Don,
Without a more directed (i.e., more hands-on) approach, I doubt you could manage to draw subscriptions from more than roughly 5% of the frequent site visitors. Pyramid only charges $20 a year, gives access to pre-publication materials for playtest, pays its writers, and generates a wealth of articles, columns and news on a weekly basis. This would require TrekRPG becoming more of a web 'zine than a message board, and somehow I doubt you have sufficient free time to take on the added role of Editor-in-Chief.
I would personally rather see a "donate what you can" approach to a fixed "subscription cost", mainly because that gives even the least well-off among us incentive to contribute monetarily. You might confer with the webmasters of various "free but we accept donations" game-oriented websites to determine which "perks" have proven most effective in encouraging the financial support of their regulars. I know The Forge has such a program in place, but have no idea how much financial support it has generated.
Of the 2,167 members of these boards, a quick advanced member search reveals that over 1,100 have either a) never posted at all or b) last posted before January 1st, 2002. That more than halves your base of potential donation contributors. It might be worth your while to email people who have not posted recently (or at all) to reconfirm their memberships, just so you can "clean house" and have a better sense of your current membership.
“In our every deliberation, we must consider the impact of our decisions on the next seven generations.”
-- Great Law of the Iroquois Confederacy