Cisco IPv6 BNG – Understanding Radius Attributes

I am trying to push out IPv6 unnumbered to an interface via radius. I am trying to do it in a similar fashion to how you would for IPv4:

Cisco-AVPair += "ip:vrf-id=vrf-name",
Cisco-AVPair += "ip:ip-unnumbered=Loopback301",

I am trying to assign a user into a VRF and of course that part is easy, but when you put a VRF on an interface it gets rid of any IP addressing, hence the need for the unnumbered interface commands. I can push the IPv6 configuration easily with the lcp:interface-config command:

Cisco-AVPair += "lcp:interface-config#1=ipv6 unnumbered loopback301",

But that has the adverse side effect of needing to support sub-interfaces:

%FMANRP_ESS-4-FULLVAI: Session creation failed due to Full Virtual-Access Interfaces not being supported. Check that all applied Virtual-Template and RADIUS features support Virtual-Access sub-interfaces. swidb= 0x7FF07FD32098, ifnum= 91

I would rather avoid enabling this functionality for the sake of the scalability drop that occurs on the BRAS. (I do know how, if it does come to that! :))

I am wondering if anyone knows a way of pushing out the above IPv4 unnumbered command for IPv6 so I can keep using full virtual access interfaces?

Note: I do know that for IPv6, I don't technically need an address on the BRAS side of things, or CPE for that matter, as it works over the link-local. In this case, I do need an address as DHCPv6 seems to only work when one is configured.

Edit: This is on a Cisco ASR platform running XE.