This is the result of a short saga on Freenode #ipv6, which broquea suggested I should post.
To make a Thomson TG585 v7 forward proto 41, issue this command through telnet:
nat mapadd intf=Internet type=nat outside_addr=<wan_ipv4> inside_addr=<lan_ipv4> protocol=6to4 mode=inbound
To view all the current NAT mappings:
nat maplist expand=enabled
Full documentation for the Telnet interface can be found here:
http://www.speedtouch.nl/docs/TG585-v7_CLI_guide_R74.pdf