...but then the Rockbox developers and users (me) have patent risk.
The API approach is a gamble, but I think it's probably a bad gamble.
With the transition from PC/Laptop to carryables/wearables, I think we're heading into a decade where a lot of free software users will be putting free software on their devices against the manufacturer's wishes.
Offering patent protection in return for documentation probably isn't enough. If software patents were that big an issue for the hardware manufacturers, they would be working on their own campaign to get that protection without having to document their interfaces. The current relative silence on the issue suggests it's not that motivating, so the gamble is risky.
(That said, the general v. special distinction also has problems. What's "special" about an mp3 player? It just does a subset of what my PC does. Is "special" supposed to refer to medical devices? How do we make that clear in a legal text?)