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