Sure thing. And, I don't think someone is going to chose a Mediocre shiny in a dive ball over a Flawless bred shiny.
So, in Platinum there are two different RNG. The Incubation RNG and the Normal RNG.
The Incubation is used for the egg. It is manipulatable in such a way that by using the Poketech to call the RNG and progress the Seeds, we can set the Seed to make it so the egg will be shiny. This RNG isn't changed when we take steps, or stand in town with all the NPCs moving around. If it was, we wouldn't be able to get the shinies because the RNG would change the seed. So, when we have an Egg to SR in the day care, we can make the egg shiny by the seed manipulation, then proceed to SR it for good IVs.
The Normal RNG is what is used for catching wild pokemon, legends, and really everything else. Walking effects it. NPCs walking effects it. Turning around effects it. Anything you can think of, really, effects it. Time in game *might* effect it. That's why we can get the same spreads on legends when we wait a little while. I did it my self with Azelf.
So, the question was could we use the IRNG (Incubation) to make legends shiny, and I'm going to have to say no. The NRNG is what calls the seed and PID for the legends we fight. By moving the Poketech around, we're changing the IRNG. It won't have an effect on the legend's shinyness.
However, I have a very base understanding of this, and could be wrong. But, I believe what I've gathered is mostly, if not all, correct.
EDIT: Can someone please tell me which Japanese link to click to get the App? I want to try getting a shiny.