User Mode Linux is very cool/fun/useful.
Unfortunately, now spamd can't get access to each user's ~/.spamassassin/user_prefs because it doesn't see that filesystem, and even if I made it, it wouldn't have necessary permissions.
So, it looks like I'll need to learn SpamAssassin's MySQL backend to get user prefs, which is actually nice. It's something I oughtta learn anyway.
But reading the spamc/spamd protocol, I'm curious why they didn't just include protocol support for the client pushing its config to the server. Configs are small. Ideally postfix would get it, switch to the user the mail's for, run the global /etc/procmailrc, which runs spamc, spamc picks up its config, pushes it to server, etc. Auto-whitelisting score database and bayesian stuff could also be stored on the server, modified via spamc.