I think the answer is the same as to my question. The moment you do it deliberately you are in trouble. If it is there by accident, a one off, you are okay, but if you are placing deliberately it to give an aid then you start to break the rules.I thought about this thread whilst playing yesterday evening. I always like to pick a spot on the grass in front of my ball and line up with that. On one fairway, the shadow of my bag was casting itself over the area just forward of my ball. It annoyed me so I moved my bag, but it made me think: would deliberately placing equipment so that it's shadow is helpful be against the rules? BTW, I'm not doing this and I'm not going to start doing to it, even if it's legal. It's just something (vaguely!) interesting to ponder.
That is my reading of things.
We just need to keep focusing on the daisy, the twig, the bare patch etc that is there naturally.