Making Emu Loader use a custom emulator to run MAME softlist games was quite easy after I ported custom emulators and emu parameters from EmuCon, even though not all MAME softlist games work with custom emulators (from my tests).
Sure, in Emu Loader you can run custom games with custom emulators in the same way you do with EmuCon frontend.
But what about running a custom game with a MAME machine without resorting to the "emulator parameters" feature from EmuCon (that only gives you 2 parameters to choose from) ?
It took me around 11 days to figure out the best way to use the arcade MAME executable already selected in the "Arcade Emulators Setup" and let the user select any of the MAME machines for a custom system.
To run a MSX1 custom game using ANY of the MSX1 / MSX2 / MSX2+ / MSX Turbo-R MAME machines. This was a very stressing and frustrating task. I almost lost hope...