The daily registrations are down from double digits to single digits, so the new questions are having an effect, if not enough of one.
The bots that are getting through are clever. They specifically ignore any field that is not marked as required to avoid parsing requirements where necessary. This behaviour doesn't work on custom forms, but most of these spambots are clearly built specifically for IPB, so they can get away with it because they understand the rules. So the best course of action is to break the rules. To do this I have just fooled IPB into marking the invisible honeypot field as required while continuing to parse it as optional. If this encourages spambots to fill in the field during rego then I can proceed further and block rego for these spambots entirely or auto ban them (only if I can't block them, since this option is less ideal), depending on the level of control hooks allow.