OK, this is far from "the easiest way", but if I had to to something like that, I'd set up an AutoHotKey script that on a certain keypress opens IrfanView, displaying an image with the same name as the MAME rom.
Set up the script to restrict all other possible control panel keys except the one that opened IrfanView, and set that key to close IrfanView and ExitApp.
Also set the script to close if no image with that name is found.
That way you can create a folder with instruction graphics that will pop up when you press a certain key and close when you press the same key. I've done something similar with the MaLa frontend and Virtual Pinball to display the instruction cards from the MaLa menu - never tried to set it for activation during gameplay though. You may have to send an additional alt+tab or something to minimize your game first.