Isn't this how ordinary spanning monitors works? It might be slightly awkward with AR goggles since the relative orientation of the displays will be constantly changing as your head moves, and what happens to a window you have half on and half off of the Macbook's screen when you look away? Or do you want to have the application jump between devices, like appearing on your fridge when you go for a drink? With the old X11 protocol and a daemon in the middle this was possible but the use cases were extremely limited and the security issues made it a pain in the ass to actually use. With distros moving away from X11 this is only going to get harder, and you have to ask yourself how much you really want it.

This would mean the goggles would be basically just a dumb display for the Mac. It would also be weird to try to move an AR app onto your Mac.