How do you do that? I have read the IPv4 protocol spec, there isn't any space to put that byte can still be IPv4. That is what I mean when nobody has proposed anything that I can implement: I have read the spec and it doesn't allow room for what you want to do. Sure conceptually you can describe any number of ideas - but they are not IPv4 and no existing computer or router will work with it - so we may as well go with IPv6 which many smart people spent who understand the real problems of the internet have a lot of effort creating to solve existing problems to the best of their ability.

Brazil didn't just add that leading 0. They planed this well in advance and forced everyone who connects to the phone system to update their systems to support it: you must apply a software update or buy new hardware; otherwise your phones stop working. Of course most of the software and hardware was controlled by the Brazil phone company (s?) and so they could ensure this was all done.

I first found out about Brazil doing this from your comment - yet I can say the above with all confidence because those are things that have to happen behind the scenes to make it work. (I have no doubt people who actually know something about what Brazil did can tell you things I didn't think about)