I have thought about this too. It is very possible that Nvidia forbids it. Or it is is technically impossible. Depends on how paranoid one is to which they guess.
Unlike desktop monitors, notebook displays won’t require that proprietary G-Sync module, but to ensure quality, Nvidia is pretty stingy with its approval process. For example, all of the current G-Sync-enabled laptop displays top out at 75Hz, not the standard 60Hz. As for supported mobile GPUs, right now it’s just the 965M, 970M, and 980M. Nvidia is dedicated to the G-Sync cause, so expect to see a proliferation of G-Sync gaming notebooks in the near future.
Nvidia tests your monitor, they do QC, they ensure that every single Gsync monitor out is up to their standards. There are multiple videos on this, and it was massively covered during Gsync's launch.
Obviously it's good for Nvidia and the customer to ensure that every monitor works the way it's supposed to. AMD's plan is to let monitors compete against each other. Nvidia wants to ensure that they get as little bad press as possible and ensure their monitors with their branding on it are top quality. Nvidia is not going to be associated with a less than premium product.