> Does Rockbox's work make the iPod's interfaces public?
No, If it did it would make everything on the iPod safe.
I would say that Rockbox documenting what they _think_ the interfaces are should protect Rockbox, but it would not protect Apple software (after all, there is no way of knowing is Rockbox got things right, or just 'right enough to work')
As for a library to hide secret interfaces. As long as the open ones are documented and can be used by others, I don't have a big problem with this 'loophole'. Either the library makes enough available for people to use it to write other software (in which case it is functionally equivalent to the 'secret' interface), or it doesn't, (in which case it can't be used for things that want to take advantage of the 'secret' commands)
I'll note that I am one of the people who doesn't consider a system non-free just because it has loadable firmware blobs.