I’m very much interested in this too 
Do I understand correctly that “screen space distances” essentially means the depth-buffer data for each pixel? This could be really useful for some advanced use-cases (estimating normals, “batch” raycasting of many screen rays at once, etc.).
By the way, would it also be possible to provide a custom view matrix to such an API? (e.g. so that fast raycasting against non-meshes could be used for non-viewport rays too)