in your onPress start a loop the checks for a hittest with your movieclips. if positive, highlight them if they are not already highlighted and un-highlight them if they are already highlighted. onRelease terminate the loop.
But onPress event will be for a particular movieclip. To detect for all movieclips wont I need onMouseMove?
as i mentioned, you'll need some kind of loop. onMouseMove is among the loop types that would work.