Can you delegate the prefixes just by DHCPv6 Server? I mean, once the PPP session is stablished with your client you run the DHCPv6 Server on top of this interface, and client will request the PD using DHCPv6 Client. In the server side, you and do then static bindings to the prefixes delegated. I don't know if that is 100% what you need, but definitely can be done. Another option will be maybe to create a PPPoE server binding that associate your client with a given prefix from this pool. Not sure if this can be done, didn't try.
Find below an example. My provider delivers me IPv4 over a PPP conenction called "internet", and then I request the prefix from this interface, using DHCPv6. Then, with the dynamic pool created, I can sub-delegate part of this prefix with my own DHCPv6 server for remote routers (provide IPv6 to some LTE devices without IPv6 deployed in the middle of nowhere). This final binding you see as dynamic in the top level tab can be made static by just pressing "Make Static" option.
ipv6-pd.png
Regards!
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