I baked the first decorative panel in Substance Painter, and made a very quick marble material. This is a first pass, as a sort of proof-of-concept to assess whether projected decals was a suitable approach for these panels.
I then created a really quick projected decal in engine to assess the result.
I also tested crating a decal using Parallax Oclusion, and although it looks cool I think its a bit needless for an asset that is so high up. I may use this for the relief sculpts that are at camera level.