What about OS X? It looks like this should work with any existing DX12 class hardware. No hardware changes required.
OS X already has a very similar capability, but developers doesn't typically use it. I believe Civ V was supposed to support Apple's multi GPU render on OS X.
I believe you mean Grand Central's use of OpenCL for discovering any compute devices installed in a computer, but correct me if there was something I missed. I can't see how OS X's oldish version of OpenGL or even OpenCL could be used effectively for multi vendor GPU gaming.
https://developer.apple.com/library...rogGuide/UsingGCDwOpenCL/UsingGCDwOpenCL.html