Nvidia already did this once before: their driver license specifically prohibits you from using GeForce cards in datacenters. Only Tesla-branded cards (no, not the car maker) are licensed for use in a datacenter.
The license is pretty much unavoidable, even if you're using Nouveau, because the cards won't work at all unless you give it's power-management processor an Nvidia-signed binary to run, and Nvidia won't sign Free replacements for that binary.
Mining operations are pretty fucking sketch for the most part (in china they just straight-up steal electricity) and miners won't care about terms of service in the same way that a billion-dollar corporation would. You're not going to get ratted out by the secretary and have all your assets siezed, short of requiring an always-on internet connection and ratting the user out for mining (which would be incredibly unpopular) there's not a whole lot you can do, it's all possible to do stealthily.
The license is pretty much unavoidable, even if you're using Nouveau, because the cards won't work at all unless you give it's power-management processor an Nvidia-signed binary to run, and Nvidia won't sign Free replacements for that binary.