If you make it a very nice looking OS project, most of the time they are either on board or don't care too much.
But I really check for this at the interview stage, and they must have a really good reason not to, compensated accordingly.
Didn't Apple start doing OS as well though? I think there are a bunch of projects from them already.
They have opened swift, but that's sort of a funnel anyway.
The real problem is folks who work there say it's really hard to open source stuff as an employee.