Has anyone had any success setting up 6to4 from a home Xtra broadband connection?

I've been playing around with Linux & Mac boxes (behind NAT) and a Cisco router (which is assigned the public IPv4 addr) and a dual-stacked (telehoused outside Xtra's nw) Linux box but all my testing seems to suggest that Xtra are filtering protocol 41 - even though my traceroute attempts with raw GRE also fail but which I know to work because of the functioning VPN using it.

I can ping 192.88.99.1 (and traceroutes appear to go to .au...) from home (...traceroute from my colo'd box reaches fx.net.nz in Wgtn) and I've even tried using my colo'd box as a relay but the IPv4 traffic doesn't seem to make it there.

The one thing that would give me certainty which I can't do is put a Linux box in the place of the Cisco router to see if any proto 41 traffic makes to the the public IPv4 address but then if they are filtering this outbound traffic then it's unlikely inbound would get through.

Any ideas/knowledge on this subject please?