Apply a redirect before checking other restrictions

[Darren Pilgrim]

I want to rewrite the envelope recipient of a message if it's from a 
specific sender, but have that rewrite change the envelope before 
reaching permit_auth_destination (i.e., an immediate, before-queue 
rewrite).  I want this so that I don't have to allow open relay from a 
given address just to bypass relay restrictions that don't apply anyway 
because the mail will ultimately redirect to an address within 
mydestinations.

The sender can't authenticate itself, so I'm a bit stuck.  Is the above 
possible?