Let's say you have a non-MPLS backbone in which you have two PE routers(maybe it doesn't fit the traditional PE definition, but let's call then customer-facing routers). You have a MP-GP session between the two PEs advertising prefixes and VPN labels.
If you configure GRE tunnel or L2TPv3 tunnel with no LDP over them between the two PEs and make sure that Loopback interfaces (next_hop attribute) are available through the tunnels, will it be possibe to send the traffic and maintain connectivity for VPN customers?
In this case the packet within the backbone would look like (in GRE case):
IP is the outer IP header for GRE tunnel endpoints.
GRE will also need to support MPLS in "Protocol Type" field in GRE header, which I suppose it does...