I'd assume:

    Vendor C #1 = Intel on Linux
    Vendor C #2 = Intel on Windows

Given the "open source wiz kids to keep driver #1 plodding forward" and "GL on this platform is totally a second class citizen" comments in each description.

... not to mention the bit about C1 being a couple years behind spec:

    $ glxinfo | egrep 'OpenGL.*version'
    OpenGL core profile version string: 3.3 (Core Profile) Mesa 10.1.0
    OpenGL core profile shading language version string: 3.30
    OpenGL version string: 3.0 Mesa 10.1.0
    OpenGL shading language version string: 1.30

Having some small internal knowledge, I agree

I can't imagine anyone taking it up, as it'd be a monstrous project, but it'd be interesting to see someone port the Linux OSS Intel drivers to Windows and/or OSX.

You'd need access to the Windows OpenGL ICD DDK which is one of the few things Microsoft doesn't hand out freely and without signing a NDA (unless the situation somehow changed withing the past few years).

OTOH the ICD API should be easy enough to reverse engineer. The registry keys, where the OpenGL ICD is registered are well known and there are plenty of drivers in plenty of versions you can dissect to learn how to talk "ICD".