PSSL doesn't need to be emulated, it's compiled offline to machine code binaries for the GPU. The DirectX -> OpenGL wined3d backend is similar inspirit, but the bytecode (DXBC) in D3D is a much, much higher level, so better compilation is possible. Untangling machine code into good shading language is not easy.

Additionally, GNM is a lot lower level than even something like Vulkan, and you can pass pointers directly between the CPU and GPU, they share the same memory space. It's this that makes emulating things tricky.

> Though I wonder how feasible would be trying to pull graphics driver from PS4 firmware

There is no "graphics driver", in the traditional sense.