Courtesy of @HyperspaceMadness, RetroArch nightly builds now have the ability to append and prepend shader presets. This makes it a lot easier to, say, put a multipass dedithering shader in front of a big CRT shader.
I have a PR in at the slang repo to make a number of shader presets (including mdapt) play nicer with the feature, but there will certainly be some growing pains as we identify presets that have (fixable) conflicts, like variable name collisions, etc. Feel free to report such issues here. I can’t guarantee that I’ll be able to fix them, but I will certainly take a look.