I wonder if an MPLS label would do the trick...
If both routers support MPLS then each bridge group's traffic could be identified by a unique label over the frame PVC. Once arriving at the destination router the label is pop'ed and the traffic forwarded to an associated Ethernet interface for example.
... or, certain Frame-Relay aware csu/dsu's also support embedded DLCI's within the Q.933 frame. This is used for PVC management, without paying for additional management pvc's, but could also distinguish between your bridge groups. The problem there will be interface availability on the csu/dsu since these are not generally handling muxing chores. I'm not sure if anyone makes a csu/dsu product that would meet your needs.
