Upon speaking with yamipoli, a respected person in the pokemon community, and someone who attended the 10 aniv event, she said that a 10 year old boy 3 people in front of her in line got a black charizard, and was hysterical about it. She said it didn't look faked and that it wouldn't have been practical to do that, AT the event. She kept going back in line to fill up numerous cartridges for her friends back home.
The theory (I don't know whether it is true or not) is that the distribution machine has a check for shinyness just like colosseum and , however it only checks to see if the PID <--> IV is shiny ONCE and then generates a new one if it is. If a person is lucky enough (1 in 6.7 million), that person could theoretically get a shiny twice and that could explain how this kid got the shiny Pokemon.
With that said, the legality that PPSE and this website will follow for events are "within the realm of possibility" for the "method of distribution". Because a distribution machine (link-cable) was used at the event, if this theory was found to be true, it is theoretically possible to get a shiny.
Right now I would make the point that it is remotely possible, but in a practical sense, the chances of one person having it, and putting it out in the wild, are slim to none.