I was a D1 college catcher, now a college umpire. As a catcher, I had zero problem with the umpire placing a hand on my back, zero. As an umpire, I very rarely place a hand on the catcher, but do occasionally to measure myself. If I do, I generally tell the catcher, "Sorry about that." I have never had a catcher even react to it, much less say anything negative about it.
Just my experiences.