Attached please find the modified nat64d binary + source code patch. It auto-detects your NAT64 subnet, so it should work for t-mobile or any other ipv6-only connection that supports NAT64. It is an ugly hack job but it works for me.

Please note the hostname it uses to detect is ipv4.google.com -- if you have a mobile operator or government who blocks or intercepts google traffic, this may cause it not to work. In that case edit the source code and change the define PLAT_FROM_DNS64_HOSTNAME in nat64d.h to some other host that only has an IPv4 IP and not IPv6.

You must have already installed the nat64d package from:
https://code.google.com/p/n900ipv6/wiki/Nat64D

then copy my new nat64d into /usr/sbin/ and reconnect to your IPv6 GPRS access point. Original source code can be downloaded from the above page as well.

Subnet autodetection code was taken from android clat whose source code is available here:
https://github.com/ddrown/android_external_android-clat

A full port of android-clat would probably be best, since it includes many small fixes and improvements over the maemo version of nat64d, but it is beyond my skills or time limit.