I want to say you can possibly see the very bottom of a cup for a brief moment, on the scout who battles Han, right when he tells his partner "Go for help! Go". I can't find a screen shot of it though.
I think the actors actually did wear them on screen, since all they had in their helmets were hard-hat liners. The ROTJ TKs had a very similar set-up & wore a chip strap.
I know I've seen a behind the scenes pic somewhere of the actors lifting up the face plate between shots of the speeder bike chase. I believe you could see the chin cup there. Sorry, I have no idea where that pic is posted now.
I also believe (and I could be wrong) that the screen-used helmets were a bit larger than the ones commonly trooped in, like KS or MLC. This may be a reason why thay can be clearly seen on some of our troopers, but not on the real actors and stunt men.