If you want just look if the system is up or down, you can easly and just connect it (with resistor =>1KOhm) to the power pin of one USB plug. Because PI switch off power on USB if the system is down. Even when power subsist on the board.
if it's possible on recalbox at all you have to be using the GenesisPlusGX core and open up retroarch's quick menu and disable FM sound under options. PicoDrive does not have that feature and it is the default emulator for sega consoles. To change the default emulator for a specific rom (I recommend this way since GenesisPlusGX is not as smooth on the Pi as PicoDrive) you go to the rom in emulation station and press select to edit the rom metadata. First two options should be set to libretro and GenesisPlusGX. This will force that emulator to be used always for that one rom. Alternatively (and not recommended) you can change ALL master system games to use GenesisPlusGX by pressing start in ES and going to the GAMES menu, ADVANCED, Master System and change emulator to libretro and core to GensisPlusGX. Here's a list of roms that support FM sound if you use the preferred method and want to edit the metadata just for that handful of roms: http://segaretro.org/FM_Sound_Unit Hope this helps Eric