That's not how that works, at all.A poster above said that the XB1S draws 50W. I haven't fact-checked.
Since it would only need to push 720p, it wouldn't need to be as powerful, so that helps.
On the other hand, we'd need to power a screen.
Bottom line: I don't know exactly how feasible or not-feasible this is. I see lots of comparisons to Switch, but that's $299 and also includes other new tech that this would not. Maybe in a few years?
This machine isn't happening.