Can Postfix rewrite envelope sender on relay?

[Gary Oberbrunner]

I know this is not Postfix's job, the MUA should do it, but because of
various weird restrictions, I *need* postfix (or something!) to do it. I 
need the my mail server (not under my control) will reject it. Postfix 
is my last hope.

Is there any way, hackish or not, for me to rewrite the envelope sender
to match the From: header? I have formail so I could use that in a
pipe, I suppose... but where do I hook it in?

thanks;

-- Gary Oberbrunner