You don't want to change the Return-Path but when a message gets returned it will go to *suprise* the Return-Path?

Why not just parse the bounces out using procmailrc just before they hit the return path address? This is primative, see RFFC 1894 for the full details on the 'approved bounce spec'. Don't expect all bounces to follow it though.

# Redirect bounces to local bounce account
:0
* ^Subject.*(Undelivered|Returned).*Mail
bounce
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