I hope that you are aware of the situation, that this sort of thing is not work of MS, it's work of tens of people as a open source software.
Cross-platform, sophisticated frontend for the libretro API. Licensed GPLv3. - libretro/RetroArch
github.com
Well it's actually more than 400 people.
That emulator is far from mature and also it's hard to say, what kind of limitation there is to the UWP app. Nobody really came forward with this. I tried back then create some podscast app and there was a tons of APIs, but I don't know if there were some APIs to call GPU/CPU directly.