I did some more research, and talked with a colleague to confirm that your findings are correct. Q-in-Q wont work with this type of connection. The 7048 cannot be configured for NAT. We did some looking around to see if we could find any other feature, but were unable to find anything that would achieve what you are looking for, sorry. 
Can I NAT to a site that lies across an MPLS link that only carries certain subnets? Our MPLS currently carries 172.16 traffic which is routed by the PC 7048 (5.1.0.1 VxWorks 6.6) We have added a couple of sites at one side of our MPLS and they are 192.168.x.x subnets. Instead of waiting to get the carrier network upgraded to carry 192.168, can I do it with NAT directly from the 7048? Failing that, any other method? Tunneling? Anything? Thanks 
Is NAT possible using a PC 7048