I think amavisd.conf operates globally.
I need to do this only for a few users.
For other reasons, I think I need MailDrop.
1. Spam settings on a per user basis. Some users do better with a spamassassin score of 3.0 versus the default 5.0.
2. Some users want spam to go to /dev/null and not clutter their inbox.
3. But others want it the way it works by default:
* Deliver everything to the inbox
* Mark spam with ***SPAM***
* Spamassassin at default 5.0
4. Users can manage their own spam settings, with SquirrelMail plugins.
So I think I need to go with maildrop unless there is a way to configure the postfix virtual delivery transport to do this.