The Xbox gpu and apu are 'off-the-shelf' in the sense that they were re-used in nvidia retail chips. But you're right, that doesn't mean much in terms of public documentation.

So you do some reverse-engineering. On the whole it isn't much work than other consoles of that generation. The only reason there hasn't been any activity is because nobody (outside of the Xbox backwards compatibility team) sat down and did the work (because there's no incentive to)