Boy, will this be unpopular...

The "legally binding pledge" part gave me the idea. I doubt you'll ever get that implemented. But what this sounds like is premium content. That is, something that people would be willing to pay for. So... limit membership to ETS foundation contributors. They mail you a check for a minimum amount, so you get their name, and in the comment area they can write their ETS forum name.

If you can, restrict access to a subforum on a member by member basis. Granted, it would be some additional admin work. On the plus side, only those members who really, really wanted to be there would be, and they'd risk being banned without a refund. Also, ETS gets more money.