I think there's a potential precedent in the reversing of the original PC BIOS - it was deemed that what was effectively a reimplementation of a closed protocol

I think there's a potential precedent in the reversing of the original PC BIOS - it was deemed that what was effectively a reimplementation of a closed protocol was permissible. And, IIRC, there are still mixed results regarding the extent to which EULAs are enforcible.