Handling parents and answering questions about playing time comes with the HC job. Only the inept and cowards hide behind "policies" designed to avoid accountability. If Sally is riding the pine and you as a coach can't handle a conversation with a parent you need to go coach the chess club. Grow a spine and do your job.
If its playing time that needs to be discussed then I don't see how parents need to discuss it with the HC at the HS age and above. It's time for the players to do it. Life isn't fair. It's a great chance for them to learn how to deal with these types of situations now.
Most coaches want to win, if that kid isn't doing it for them there probably is a good reason they are on the bench. Just because a policy is in place doesn't mean a coach can't handle that discussion, the policy just shuts the door to any of the parent BS or potential confrontational situations where the parent blinded by their love for their child becomes unreasonable. I'm not saying you are, but there a definitely a lot of crazy parents out there, probably a lot right here on DFP.
If a HS age player is on the bench that player can find out how to get more playing time themselves without the helicopter parents having to do the talking for them. I know my kid would be embarrassed forever if I had to talk to the head coach for her about playing time. She would handle it herself.