They're kinda sorta starting down this path, as they announced a new GitHub org (swiftlang) this week, which will eventually be the home of Swift and all its related libraries, including the rewritten Foundation. This should allow outside contributors direct commit access much more easily, and allows the language to start moving more out of Apple's control.

I don't think they're going to let you claim a donation to an open source developer as some sort of charitable donation, which means the tax write off would be no different here from any other business expense.