I can handle single emails sent out to individuals, just not all members at the same time.
Ideally, each forum subscription or watch list would send email to it's own email discussion list (ie one email, one address). Distribution of email to users would be managed by the discussion list (ie mailman). The biggest problem here is how to automate the subscribe/unsubscribe to each forum discussion list. I think it's possible, but would require sending a command to the mailman admin
http://list.mydomain.com/admin.cgi. This would be separate from the UBBT database.
Either that or manage the subscriptions manually.