It wouldn't be hard, but who would pay scalper prices for something that they then have to run dodgy software on that may or may not jeopardize their entire steam account?

This is a different threat model than anti-cheat. Here you just want it to be annoying enough to stop scalping.

So presumably if you make an account which didn't buy a Steam Machine unable to log in on one, you kill the scalping market. It doesn't have to be perfect to make it unpalatable for scalping. Is a scalper going to take the risk on buying a ton of hardware which they can't offload at a profit without also getting users to hack the thing to get it to work and risk a Steam ban?