fMSX core added support for .m3u recently (and much improved SCC sound cartridge emulation), which is already in the latest core update from buildbot:
But “fmsx_libretro.info” on buildbot (inside https://buildbot.libretro.com/assets/frontend/info.zip) wasn’t updated and doesn’t include .m3u in “supported_extensions”. This prevents the core from loading .m3u or showing “Disc Control” in Quick Menu.
I manually added .m3u to “fmsx_libretro.info” and tested a few multi-disk games on Windows and Android, including “Dead of the Brain”, “Snatcher”, and “SD Snatcher”. m3u loading and disk swapping both worked correctly in these games. Hope someone can update “fmsx_libretro.info” on buildbot soon.