My impression from reading is that Zen is more particular about posture than most.
Yes, In my Tibetan Buddhist sangha, one of our members who used to do Zen has the best sitting posture. He's always seriously sitting very straight and perfect in his quarter lotus posture--but now with his hands on his knees because my sangha uses this manner of sitting which comes from Marpa.
Maybe the idea is to take posture itself as the object of meditation, who knows?
I've never had a Zen teacher, so I'm not qualified to comment on Zen, but my lama taught one method of shinay which was exactly that. You simply focus on maintaining all the points of an ideal posture. When you deviate from them, you are aware and fix them. No special focus on your breath in necessary. I wonder if the shikantaza method of Zen is similar to this method of shinay?