How about this:
- Players may call for the trainer and take a MTO before their own serve for any condition that has arison during the match.
- If a player wants to call the trainer before their opponent's serve they must forfeit that game.
- If a player requires restrapping or taping of an existing injury they must forfeit the next game.
- The umpire may call for the trainer immediately in an emergency
The last one is essential. Any restrictions that you place on players requesting MTOs should not endanger the player. If the player falls and hurts herself, is hit in the face by the ball or passes out, the umpire should be able to call the trainer immediately. But if the player requests the trainer, then the first three rules apply.