If there is no fix pattern for the username field in RPC or if its encrypted (via NTLM hash etc.) then there is no chance you can match it using regex.If the field is predictable, then a custom signature can be made as desired.
The MARS box does not do any detection or regex matching itself, it just correlates the signature events received from the IPS, so I doubt MARS would help here.
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...