Re: With source to drivers, will it now be a walk in the park for the other OSes?
Did you try booting into Windows with driver signing enforcement disabled and then installing the unsigned driver(s)? That's how I got my modified nVidia driver for my laptop to work. I had to change the PCI ID in the .inf file to work with my card, and of course, any modification causes the driver to become unsigned. It's working fine, though, in 8.1 x64 (it worked also in 10 x64 when I was using it).
Once it's installed, there's no more signature checking; it appears to only be done at installation time.