There really isn't a direct analogy to proxy arp in v6 because you're dealing in subnets with V6 as opposed to individual host addresses.
How are you routing the /48 to the PPPoE servers?
You will probably need to redistribute static routes into OSPF, but you could keep this minimized with some filters.
As long as each PPPoE server has an assigned subset of this 48 for user pools, say you split the 48 into 16 /52 blocks - this would let you have up to 16 PPPoE servers. On each server, black-hole route its own /52 and then redistribute static routes - allowing the /52 through the filter, but discarding any sub-ranges of that same /52 (ospf-out filter is used in IPv4 to accomplish this, so I'd expect the same to be needed in OSPFv3)
If any user "roams" to a different server, then their individual prefix will be allowed as a more specific route.
When given a spoon,
you should not cling to your fork.
The soup will get cold.