Some experiments for getting SSR to reflect diffuse materials for SSGI like effect. Works in viewport in Eevee. Just replacing principled shader with node group that has a gloss shader that uses the some of the same inputs and roughness mapped to a more diffuse range and then optionally mixing 50% to a empty shader to same brightness level as normal principled shader.
SSGI with no brightness change from original:
SSGI with spotlights and emissives:
Lighting from one emissive mesh:
And the node setup (some of that is unnecessary, some some setups work better for scenes, some for skin):
I was hoping to create a addon that would replace all the principled BSDFs with the nodegroup, but it was bit more complex than I imagined. Didn’t help that most of the relevant examples of similar scripts were pre 2.8.