well, if you want to add an extra byte you kinda have a problem, since v4 is fixed format and is actually cooked into hardware in a lot of places. so if you want to keep v4 mostly untouched you have to use an option, which is going to be pretty slow on the backbone.
you can send a packet from an extended address host to a vanilla v4 host if you map the address space into a range like you suggest..but that v4 host just has no way of sending a message back..so its kinda useless
It'd be useless until everyone switches to the 5-byte thing and people can start putting something besides 0 into that last byte. But at least they could turn on v5 or whatever it's called without having to think about it. Right now I could have two hosts that both agree to use ipv6 and it's still hard because you have to reconfigure everything.