It's currently a restriction because otherwise the ABR doing the summarization will receive packets with VPN label only due to PHP. Traffic will be lost as the ABR is not aware of the VPN labels.
That's why you need a LSP between the PEs directly.
please read the article given
Now loopback summarization is possible, for this please read the given post
There is also another approach via draft-swallow-mpls-aggregate-fec-01
This solution doesn't have the LFIB convergence issue of RFC 5283 but introduce a third label (aggregate label) in the stack.
Is RFC 5283 supported in IOS now? I can see the upstream router receiving label-bindings for more-specific prefixes and the summarized prefix from the downstream router, but it wont assign local labels since the more-specific prefixes are not present in this router's routing-table?
LSP is built between host routes. And for MPLS L3VPN it's built between BGP Router-IDs . Summarization is not recommanded on the core to allow MPLS to form LSPs based on the OSPF link-state database..