The translation layer isn't where the interesting bits are. The parts hardware companies want to keep secret are the hardware-specific parts, not the OS-specific parts. It might even help them to open source the translation layers because then others could potentially use them and shoulder some of the maintenance cost.

I can't speak to the legal status of GPL drivers for Windows, but several seem to exist already (e.g. Windows ext4 driver), and if they were actually worried about it they could always get explicit permission from the copyright holders of the relevant code. Either they say yes and you're fine or they say no you know what pieces of code to replace.