The ump doesn't have to declare an intentional drop while the play is in progress. All the ump has to do is declare the ball dead, declare the batter out, and then return the guy on first back to first base. The ump could let the defense finish the play, declare the ball dead, and then enforce the rule. Or, the umpire could let the play finish, convene a meeting with his crew (to solicit their opinion), and then enforce the rule. Either way, the outcome is the same.