Given a prefix delegation setup like so:

ip6 nd address autoconfig GigabitEthernet3/0/0 default-route
dhcp6 client GigabitEthernet3/0/0
dhcp6 pd client GigabitEthernet3/0/0 prefix group hgw
set ip6 address loop0 prefix group hgw ::2/64

We need to send RA's with for the delegated prefix on e.g. loop0, so that clients can autoconfigure ipv6 addresses on the delegated subnet. At the moment, this is not supported. One has to resort to manually configuring RA with the delegated prefix, learned after-the-fact by "show interface addr"

ip6 nd loop0 ra-managed-config-flag
ip6 nd loop0 prefix <delegated-prefix>::/64 86400 86400

This needs to be handled automatically, in such a way that the RA advertised lifetime is correct, and so that the autoconfig flag is set in the RA.