This is an opportunity to educate, especially during fall ball. Grant the TO. Dust off the plate and let the catcher know the runners weren't going anywhere.
Between innings, go to the coach and explain there is seldom a situation where time is needed and most umpires are not going to grant it. No different than the infield requesting it or a runner diving back to the bag on a pick off attempt.
Not only does this give them an opportunity to work on it at practice, but maybe you earn some respect in the process without coming across as a OOO.
In your situation, when the catcher shrugged his shoulders, I may have took that opportunity to talk to the HC then.