I don't think this has been suggested yet.....
I don't know how technical this would get, it seems like with the subscription requirement in place it may not be hard to do, but what about a trial membership? Alot of sites do this, just as an example those sites that will let you access your credit report for a month with weekly updates. For Interference it could be a day, week, whatever time frame - they would have to initially register with basic info like they do now, and during the trial period they could read, post, whatever - all access to be able to get a feel for everything that is here, including PLEBA, LS, FYM etc. When they then go to log in after their free trial is up, they get a screen that says, times up, get the full package for just $12 a year. I'm assuming you're going to have to have a tracking system and notice like that for when paid subscriptions expire anyway.
I know there could be few potential problems, but it seems like they would be doing alot of work just to get around paying $12/year. For example, it could be a problem that people just keep re-registering with different names to keep getting it free, but you could require a valid email address to receive a password that is good only during the free trial (ie click on a link thru their email), and then when they sign up and pay they could choose their own password. Someone might sign up just to post spam, but that has already been an occasional problem here.
This may just be an option to try if there doesn't seem to be many people subscribing that aren't already on Interference. I do like to see new 'faces' around here in the forums that currently won't be available without subscription. This could help too with out-of-place postings that may become a problem as BC previously suggested with allowing free posting in only 6 forums.