My point of view is: if you are using a proxy, or a Tor exit node, be prepared to it being blocked for a duration of a spam attack ongoing through same proxy (or Tor exit node). To make matters easier for Tor users, the blocking could be of read-only kind: you can read forum, but you cannot log in aka send posts or private messages through it.

Since Tor's purpose is anonymity, logging into an account while using Tor doesn't make much sense, anyway. Right?

And if anonymous users were allowed to post without logging in... How would forum be protected from spambots? Registration of an account includes many "are you human?" checks, and spammers still like to bypass them (even if they have to use actual human-time to do it). Would you still enjoy anonymous Tor access if you had to jump through half a dozen different captchas for every post?

I would run a Tor exit node. But I would have to get a 24/7 server dedicated to it, first. And research the local implications of running it (high traffic? restrictive laws? whatever).

Tor isn't blocked here "just because it's Tor". Tor is blocked for duration of spam attack using this particular Tor exit node. It doesn't make sense to whitelist Tor exit node just because many other-non-spammer people are using it, unless you find a good way to protect forum against spam-attack (which doesn't involve too-many-human-hours of work from moderators). It could make sense to implement particularly nasty captchas-before-making-a-post against this particular IP for the duration of attack from it, if it's possible.

Just my personal opinion. And yes, I would support addition of https access to TMO.

Thank you. Best wishes.