RFC1347 (TUBA) is architecturally not very different from a Dual Stack IPv6 setup with 6rd and IPv4 (CG)NAT, except that you're stuck with the fragmented IPv4 routing table forever since the extended (CLNP) packets are routed over the original IPv4 network. And of course it's missing various other improvements made in IPv6 such as the higher baseline MTU of 1280 bytes (vs. the IPv4 baseline MTU of 576 bytes minus the CLNP headers), the removal of in-route IP fragmentation, the streamlined IP header, etc. Implementing TUBA would still requirement changes to all existing network applications and many protocols to work with extended addresses, so you might as well just add IPv6 support instead.