The open-source Nouveau driver now has "Reverse PRIME" support.
There's already been PRIME support in place so that NVIDIA Optimus laptops with Intel and NVIDIA graphics cards could have the NVIDIA rendering scanned-out over the integrated Intel graphics. The Reverse PRIME implementation is the opposite: the NVIDIA GPU can now scan-out what's being rendered by the Intel graphics processor.
The Reverse PRIME support came fairly easy with the other pieces of the puzzle in place and happened with this commit
to the xf86-video-nouveau X.Org driver.