Did a bit more mail system hacking, and wrote a filter to
get mailman to talk to SpamAssassin directly:
http://sourceforge.net/tracker/?func=detail&aid=534577&group_id=103&atid=300103
This one asks spamd to score each message that
is posted to a list. If the message goes above a particular
score (configurable, default 10), it gets discarded. If it
goes above another threshold (default 5), the message gets
held for moderation. It probably needs a bit more tweaking
to skip posts from listmembers (among other things). I
mainly wrote this patch because the PyGTK
mailing list currently gets more spam than real messages,
which is a bit depressing, and a pain to moderate.