Your local tac-shop guy was either confused about what you were talking about or doesn't know an elephant from a stick up his butt.
I have a grip plug in my G27 and it does exactly what you were mentioning. Plugs up the open part of the grip that has no function whatsoever other than to make the grip the correct size on the outside.
You can see it here behind my Pearce +1 grip extension; it's the little plastic semi-circle:
