Its a protocol that was in the works about the same time krh decides "Lets write Wayland!" Michael did an article or two about X12 around the sametime Wayland picked up.
As I understand it, X12 is the name of an protocol that has not been specified (yet, and probably never will be). So far there have only been discussions of how X12 would look and work if it were ever created.
Discussions about what should be thrown away, what should be fixed, what developer would like to have in a different way, and what new stuff they would like to see in it.