Wait, you wanted to extend the text representation of v4 addresses? That's not a thing that exists in the protocol. The addresses are in binary in the packet format and in all data structures, so any extension of the character set in the text representation has to be implemented by increasing the bit length of the addresses.

I don't know why you think this is "inherently backwards compatible" yet think v6 isn't. It's just as backwards-compatible as v6 is.