This now looks doable with a 2nd plugin running spamc. According
to the man page:
-u username
This argument has been semi-obsoleted. To have spamd use
per-user-config files, run spamc as the user whose config files spamd
should load. If youre running spamc as some other user, though, (eg.
root, mail, nobody, cyrus, etc.) then you can still use this flag.
So, I can submit to a per-user account name with this option until
it gets pulled.
Or, I can set up sudo to allow qpsmtpd to run spamc as the
per-user account from the get go.
Either way, it only requires a very minor alteration to a copy of
one of the spamassassin plugins, and it should be ready to go.
Thanks for the comments and suggestions...