How can I pass a username to spamassassin from smtp-vilter?

Question

I'm running a mail server using sendmail as the MDA, passing to Dovecot's deliver as the LDA.

I've added smtp-vilter as a means to run spamassassin on incoming mail. This works well.

However, smtp-vilter doesn't pass the username to spamassassin, which prevents spamassassin from reading user_prefs:

Jun 13 22:58:57 hostname spamd[1277]: spamd: checking message <E1QWF9r-0006B9-PR@xx.xx.co.uk> for (unknown):50

So, my questions are:

1. Is there a way to pass the username which will recieve the mail to spamc?
2. If not, is there another milter which allows tagging only which can call spamc with the username?
3. If not, is there a way to use procmail server-wide to call spamc, but keep dovecot deliver as the LDA?