PS: I don't believe that Arduino has any special provisions for 3rd party developers, in terms o things like "early documentation of directions" or "warnings about future changes." Some of that happens on the mailing list (you know about the "Developers" Mailing list already, right?)
But given that folks like Adafruit and PJRC are "frequently" seen asking stuff like "can we please get our improved xxx into the standard core", they don't seem to have any "special standing."
(maybe that's not true. The IDE support for adding 3rd party boards and packages is pretty much phenomenally better than most other IDE vendors; especially ones with their own hardware products. I'd suspect that there was SOME behind-the-scenes discussion of how that was all going to work. But I don't know.)