OK, I’ve hacked MovableType 3.0D so that if someone uses the same combination of name, e-mail address and URL (if applicable) that has been used for an approved comment in the past, their comments are automatically approved. That should prevent the comment spam from showing up, while making it convenient for the usual suspects commenters.
That oughta do it.
UPDATE: If anyone is interested in how to do this hack, just ask.