IIRC, last time I messed with it you were required to have a dedicated separate key for each slot to save or load from.
(vs being able to navigate slots with the joystick)
It's been a long time. I could be remembering wrong, but I do remember that the way it worked was not conducive to incorporating it into a cab.
Has the way any of that is handled changed in newer versions of MAME?
(my cab is on v.146)
EDIT: Does it also work for the console emulation in MAME?