You are correct. Conceptually, I dont see a problem. As far as the GRE Source and Destination IP Addresses are part of the interesting traffic between the VPN Servers, the IPSEC Tunnel as well as the GRE Tunnel should work fine.
Below is an URL that discusses a similar scenario with the Pix Firewall.
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...