I would assume that anything not published by MS would need permission from license holders. And until MS has the power to force them into voluntarily reducing their sales by creating one market for two systems (Xbox and PC), they would be stupid to go along from a business standpoint. Same work making the games for less sales, all to benefit MS.
As an aside, while it would be cool for consumers, I think the benefits would somehow end up screwing gamers over big time. MS has been talking all nice and rosey these last couple years, but that is solely because they are still trying to make the best of a bad situation in terms of console market share. Underneath the surface is a ruthless corporation that would love to kill off the competition.