An Intel engineer opened the pull requests today for adding Intel XeSS support to Valve's Proton for Steam Play along with associated changes to VKD3D-Proton and DXVK.
The proposal is to add a built-in XeSS library (libxess.dll) that forwards XeSS Direct3D 12 calls to the Linux XeSS library so that the XMX accelerated path on Intel graphics hardware can be used with Proton rather than the slower fallback codepath.
On the VKD3D-Proton side is enabling the Vulkan extensions and features required by XeSS. VKD3D-Proton's interoperability interface is used so Direct3D 12 objects are converted to Vulkan. THe DXVK change is for no longer hiding Intel GPU brand advertising when XeSS is provided by Proton.
Those curious about this Intel XeSS support for Valve's Steam Play (Proton) can find the details via this Wine (Proton) ticket. It isn't merged yet but given similar work for upscaling tech on AMD and NVIDIA graphics, this work will presumably be merged in due course.