It seems I'm not the only one doing visualizations of my spam flow in an attempt to figure out a solution (my various attempts).

I have also been investigating my proposed solution (challenge/response) given my situation (several thousand messages a week). Some searching on freshmeat.net turned up Email Secretary, a Perl/MySQL/qmail/vpopmail implementation. Except for the latter dependency, this matches my set-up, and thus getting it running should be doable. The fact that it's written in (reasonably clean) Perl will also hopefully make it easier to make the tweaks that I want (automatic deletion of messages with a SpamAssassin score above a certain threshold, white-listing based on more than just sender's address, etc.)

I have also found a few essays on challenge/response systems, which seem to contain enough guidelines and real world experience to know what to avoid.

Finally, as the image shows, it appears that, as a respected scholar, you haven't really achieved the acclamation you deserve until your name is used to dilute the spam quotient (and baffle the readers) of junk mail.