Thanks!
SubsurfaceColor worked a treat 
The spheres in the image below have a different BaseColor and SubsurfaceColor, and by accessing SubsurfaceColor in PP, I can render SubsurfaceColor to a separate target while keeping the main scene unchanged.
I guess this means I can’t use SubsurfaceColor for any rendering in the main scene, but it beats having to create a second copy of all assets, or editing engine code to create a new custom shader.
