A variant on it would be to give the GM an a break, or even a free account, at some some player threshold. Not sure, but its worth tossing around.
Earning a free or discounted GM account would be nice, but I'd like to point out it means little to you or the GMs if the GMs have a hard time recruiting players. Any tools you provide for that will benefit both you and the GMs.
As a GM, I'd rather pay my $27 (or $36) a year and be able to get 6-10 reliable players than I would to get a free account and only get 2-4 players. Of course, I'd obviously be even happier to have 6-10 players and a free account, but I want to make sure you know that from this GMs perspective I'm not worried about the cost for me nearly as much as I am about how to convince my players to play and pay.
- short term gift accounts - 1-6 month gift subscriptions that can be given by GMs.
- session passes - 2-6 hour session passes that can be given by GMs to allow someone to test their game. A few available for free, additional ones at a small cost
- visitor accounts/status - where people can watch, but not participate
- "guest" characters - allow GMs to set up special NPCs that would go into a sitewide common guest character pool. Someone interested in the site could be randomly assigned one of these guest NPCs/campaigns, and allowed to play it for a session/week/etc.
- group package discounts - a GM coordinated buy that sets up all the player accounts at once for a group (with a minimum size, of course), possibly at a discount, maybe with a shorter time frame - IE, $60 gets me 6 accounts for 4 months - after the initial four months, each player has to renew their own account at the regular rate.
And I've run out of immediate ideas...maybe more later.