I was thinking about ways of handling nat64 on the handset that works with tethering. I came up with something else (incompatible with the libnat64 library):

http://code.google.com/p/n900ipv6/wiki/Nat64D

As a bonus, skype works now. Also, fmms in havoc mode should work with it.

I still need to update the ipv6-share script to work under this, but it'll be possible.