After talking with Jim about it we both agree that this is a huge undertaking.
You would need either static addressing and RA advertisements on all PPPoE/PPtP server interfaces.
Added to this you would also need to start a DHCPv6 server on PPPoE server interfaces.
It's configured just like the rest. But not undoable.
Basically, if we can make the PPPoE server tabs show up on the DHCPv6 Server page (or a subrow of tabs) then it would be easier. not Easy, easier. As that would allow us to do both.
For PPPoE you would need to support prefix delegation ofcourse, because it could be a router or a host, we don't know. Still don't know if we can use the same prefix on each real pppoes server interface or that ISC dhcpd will balk at that.
There is also the issue that the current ISC dhcpd does not yet reclaim prefixes. So if they are all used, it will encounter and stay in a out of resources condition.
A note on the prefix delegation range on the dhcpv6 server page. The range must be started and ended on the boundaries of the size you are trying to delegate.