There have been a lot of people who have voiced opinions saying that FR ought to go to a subscription model, charging a fee (monthly, annual, etc). I believe that a straight subscription model would probably be the death of FR. I don't believe you'd get enough subscribers to keep vibrant discussion going, nor enough money to support bandwidth requirements even if you did.
While there is no doubt you'd probably have a fairly decent conversion to a membership, it would probably be in the single digits percentagewise. Most Internet businesses that have converted from a "free to fee" subscription for their content convert less than 5%, regardless of what the subscription fee is.
Many I am aware of, including some that provided extremely valuable content for Internet marketing (not the same old, same old, but great fresh content) converted in the 1-3% range. And the subscription rate was less than $20 a year for daily free Internet marketing content.
In addition, I think you would have the same problems with subscriptions that you would with commercial advertising. Namely that charging a subscription fee for access to "Fair Use Content" might not be considered kosher by the powers that be.
But, all that being said, you might provide various membership levels based on donation levels and usage. IE - to post an article you have to have a membership (and double that if you want to post a vanity;-) that you earned based on the donations you've made. This might keep some of the disruptors, DUers, AFers, and other inane posters down to a tolerable level.
You could do this and still keep your donation model. You could have membership levels depending on how much you donate. Red memberships receive certain privelages (posting articles and vanities), White memberships receive certain privelages (posting published articles only), and Red memberships (can reply to postings, but not actual do any posting themselves, access to FReepermail & Profiles, etc.). Those who don't contribute any donations still have access, but they aren't members and can only Lurk.
I kind of made this up on the fly, but I think you might have pretty good luck with it. It would probably encourage increases in donations, and still accomodate those who won't contribute now, but might later. And, I believe it would get you around charging for access to Fair Use content.
What about an e-Zine to keep folks informed and involved? Your current membership is much larger than those who visit your board on a daily or even weekly basis. If you had an e-Zine that was e-mailed out, say weekly, with events of notice, maybe a "vanity of the week", notifications of financial status or fundraising threads, weekly site statistics, and some "sell/enthusiasm" to get the lesser engaged more willing to make donations.
It's been so long since I signed up that I don't remember if you collected e-mail addresses when we registered or not. I can't believe that you wouldn't collect this information, but e-mail follow up is the most powerful tool in Internet marketing, that you've got to be willing to use it.