So - tried this - got an error on the core startup saying it couldn't save the cfg file, but it was pointing to the custom core.cfg file address. However, when the core loaded, the controls didn't work and the video was shrunk.
Any ideas? It would be great to be able to have a per-game shader setup for each game, so when you re-load that game, the shader loads with it. Any retroarch/recalbox experts in the house?