I thought that dapples were more about health too. I am just finding it exciting that every year Phoenny's dapples are becoming more pronounced. I am not doing anything differently this year than I was doing last year. I don't feed supplements at all. She gets no food other than grass and I only groom her when I get her in for a ride. In the pics that I posted I hadn't been near her with a brush for nearly two weeks.
I thought that maybe it was the fact that she has a genetic component ie a dapple grey dam that is the contributing factor to her dapples.
Here is a pic taken last year I think, as you can see she is shiny but not dappled. Maybe you can see the slightest dappling on her flank but that has been the extent of it up until this year.