Carcassonne -- problem telling the difference between the Big Meeple and regular Meeples
I've mentally lost track of big meeples before, so I don't see a problem with visually altering it to make it stand out.  Perhaps you could borrow a hat from a set of Legos?  Honestly, I'm more surprised that your gaming group is cutthroat enough to need a rule to force people to announce when using big meeples.  Carcassonne is a game of perfect information (other than tile draws).  You should always be able to ask where something is.