This is a WIP. Notes so far.

At the moment, this results in an error from DHCPv6 Client when trying to assign an addrss to ppp200:

Feb 10 23:28:50 tank dhcp6c[1063]: transmit failed: Network is unreachable

Manually adding an address makes it work:

Feb 10 23:28:47 tank dhcp6c[1382]: request prefix is 2001:44b8:7206:d180::/60
# ip -6 a a 2001:44b8:7206:d180::1/48 dev ppp200
# ping6 2001:44b8:1::6
PING 2001:44b8:1::6(2001:44b8:1::6) 56 data bytes
64 bytes from 2001:44b8:1::6: icmp_seq=0 ttl=59 time=27.8 ms
64 bytes from 2001:44b8:1::6: icmp_seq=1 ttl=59 time=27.3 ms
64 bytes from 2001:44b8:1::6: icmp_seq=2 ttl=59 time=27.6 ms