To my recollection, if a fielder attempts to catch a batted ball before it hits the field, but then misplays it AND it falls into fair ground, then it is a fair ball.
I just witnessed a high pop up in foul territory being fielded by the catcher. The batter was not legging it out to first. Given the situation posed above, couldn't the catcher slap it into fair ground, then either him or a defender backing the play up throw out the batter, granted he would still be strolling unawares?