I don’t know, I just sort of assumed that the painstakingly preconfigured artwork system MAME is already set up to use would be an easy target when it came to RetroArch’s overlay system. Since shaders in RetroArch affect the entire output – including, of course, artwork like bezels and overlays – actually using MAME artwork as artwork is out of the question, and I thought that was the whole point behind the overlay system.
But perhaps someone has either mass-converted MAME’s artwork or developed a system that achieves the same thing?