MailScanner/Spamassassin and postfix w/virtual hosting

[Webmaster]

I hope this is the correct place to post to...  This may need to go to
Spamassassin....

I currently have postfix set up to use virtual mailbox mapping, so the
users on postfix are not users of the system, and the user logins are
user at domain.  I make use of Spamassassin to check mail, and I have seen
that it is possible to have a user preference file, so the users can
override my default spam detction options (user_prefs).  this is to be
located in the user home directory of the system, but of course there is
none with virtual mailboxes.  Is there any way that I can accomplish
allowing individual preference files?

Brian