I'm the R and I yell "mouthpiece" or "chinstrap" as needed. And I always look at the punter, kicker, & holder if they sub in.
This was an experimental rule in Ohio last year. If a player is legally equipped when the play starts, we do not send them out for a play if their jersey come untucked or the jersey sleeve or pant leg roles up during the play. We tell them to fix it. If we have to tell them 3-4 times we will probably send them out to get a uniform that fits. Did that twice last year and had no complaints from the coaches.