Floob has it right. I am on 3beta on a pi2. I first tried remapping the exit combo (that another person recommended) and it did not work.
However, remapping the hotkey by adding Floob’s line to the retroarch.cfg files for fba-lr and imame4all-lr work for all games that the [rSHIFT] keyboard button had been working for to add coins.
Getting MAME games to play nice is not for the faint of heart.