When they are making the IPSEC connection with a source IP of their public address, they are in ExternalZone - but that's true even once they've gotten an Office Mode IP, and their originating address changes?
I think I might have a workaround. If I create a rule that allows all traffic from VPN clients and apply that only to the remote access VPN gateway, I can create an interface on the adjacent DMZ gateway, assign that interface to "VPNZone," and then use that zone in policy, as all DMZ-bound traffic from Office Mode clients will be coming in on that interface.