ok you mean /recalbox/share_init/system/.emulationStation/es_systems.cfg
I made a copy in /recalbox/share/system/.emulationstation
but once restarted, i do not get in emulstationmenu/.. advanced options/ mame the choice with two values defined here for instance :
<emulator name="libretro">
<cores>
<core>imame4all</core>
<core>mame078</core>
</cores>
</emulator>
so i am not sure its actually read...
btw i've found where the command is composed and launched , redirection of standard output could be put here..https://github.com/recalbox/recalbox-configgen/blob/4.0.0/configgen/emulatorlauncher.py#L151