I was trying to map 0 and 1 to the l2 and r2 bumpers.
I added these lines to /opt/retropie/configs/videopac/retroarch.cfg above the inclue:
input_player1_l2 = "num0"
input_player1_r2 = "num1"
and tried going into the emulator's "Quick Menu / Controls" by pressing Start + X
lr-o2em Controls Menu
and no luck.
There is just no option for anything other than direction and fire. the retroarch.cfg changes I can see in the menu, but don't actually work to select a game