I have checked. I have Start buttons set as MAME default (5 and 6). On JPAC, holding 5 acts as shift. I have a shifted quit key mapped to Start 2.
If there isn't a way of setting Attract Mode to only quit back to menu after the quit button has been held for a few seconds then I will move the button to somewhere else on the cab.
Thanks for you help 
First, if you use a quit option in the emulator (MAME in your case) there is no need to set an exit emulator button in AttractMode. I can only think of a handful of emulators that this option needs to be used in AttractMode. This can sometimes be more hurtful then helpful and shouldn't be used unless it's really needed (i.e. no other way to exit the emulator).
Second, if you want to hold a button for a period of time to exit an emulator (or anything else) you can use a third party tool like JoyToKey or Xpadder. These will make your setup a little more complicated but will give you maximum flexibility on controller/button setup per program/emulator.
Third, it's ideal to have non-gaming buttons on you cabinet. These buttons are never used in games and are not located near the game play buttons. I use these for the game start, game end type of function.
Fourth, if your cabinet has side buttons (normally used for pinball flippers) these are good to use as an exit button (preferably in the emulator vs AttractMode, if possible) to exit games because they won't be used in any game other than pinball.