Could really lock it down and make it only available to donating members.
$20 really isn't much to become a part of a fantastic forum.
It would work, but a few problems I see with that (being a devils advocate and all):
1. It's no longer a 'donation'. JC is now charging for a service of sorts, because if you want to post your ride, you have to pay to see the section you post it into. Besides potential tax and legal implications from what would be perceived as income, it leads into the next point....
2. The great thing about the Internet is the freedom of information and information sharing, that's what it was founded on. Pretty much all the best and biggest forums and information sharing sites are free, any information I've shared in my lifetime has ALWAYS been at no cost. Folk who run these sites may never have expected them to grow so much, but I doubt they went into these things as a business venture and in most cases wear the cost, but as JC does, seek voluntary donations where they can. To me you accept that cost wearing exercise and all that may come with it when you kick something like JC's off. Forcing a charge might not work positively to the image JC is trying to portray, ergo making it less of a fantastic forum, worst case, driving traffic away/down.
3. A quick glance of the member-list suggests only a tiny fraction of the community donates and there's 8,445 threads in the 'Post your Ride' forum. It would surely be a nightmare to deal with, if suddenly 90% of posters couldn't access their own thread.
Personally I'd consider a donation if it could be done anonymously (can it?), without some status attached, like a pat on the head, no offence to donating members intended.
IMO it would pay for JC (the board?) to be pragmatic and considered about a way forward.