master.cf variables (feature request)

[Michael]

I would like the ability to specify the destination username by 
variable when calling an external program (like Spam Assassin) in 
master.cf.

Postfix knows what the destination username is because it looked it up 
when looking up "virtual_mailbox_maps" DB.

At present it's assumption seems to be that all mail is delivered to 
usern...@example.com - so I have to call Spam Assassin via a shell 
script which does an SQL lookup based on email address.

Feedback please?

Thanks,

Michael