when selected as a 6 to 4 tunnel the only options for WAN side are:
Remote 6to4 Relay Router
Auto
Static IP Address 234.234.234.234
Doesn't look like I could use that with the tunnel broker.
doesn't allow setting details for the ipv6 address, only the last 4 (of 8) parts on the LAN side
That's a bit daft. If they have 6to4 they already have most of what they need to do 6in4.