The only way to achieve something like that is to play around with the NAT. Even though it usually works, as its not officially supported (atleast yet) theres naturally always a risk that it stops working because of some change.
I have tried this for example in a situation where the user had 2 ISP links and wanted one DMZ network to use the other ISP for all outbound connections while the rest used the other ISP.
Table of ContentsIntroductionVersion HistoryPossible Future
UpdatesDocuments PurposeNAT Operation in ASA 8.3+ SectionsRule Types
Network Object NATTwice NAT / Manual NATRule Types used per SectionNAT
Types used with Twice NAT / Manual NAT and Network Obje...
Table of Contents Introduction:This document describes details on how
NAT-T works. Background: ESP encrypts all critical information,
encapsulating the entire inner TCP/UDP datagram within an ESP header.
ESP is an IP protocol in the same sense that TCP an...