Again another awesome quest
My thoughts
During a special event golden tickets will be avaible as drops by poke' loot, random egg ect (as per Mr_Dark) - also get one from battling gym leader as a special bonus
In order to go to the island you must complete the quest submitted by Kunhs "The Shady Thief"
From that quest you will be able to use the New Pokeball - which was created in secret, for capturing these shiny pokemom.
The owner of the factory will also explain the primary use/design of the pokeball, not just for capturing difficult pokemon but for a better chance of nabbing a shiny
The owner will also reveal that there is a hidden island and you can only get there by boat and must pocess a golden ticket.
People who obtain the tickets could either keep these until they fininsh the "The Shady Thief" quest for the next event , or sell/trade them for other items

I guess you can buy as many of these special pokeballs as you can afford.

Once on the island and ready to catch.
You will be able to catch as many shiny's as you can, - maybe with other pokeballs as well but lesser sucess
Once you have used your pokeballs or happy with your catch, you can get ready to battle
You will then select 3 pokemon (or whatever is decided) and then battle the Sharman and win.
If you win you can only choose 1 pokemon to keep, and the rest are set free.

This will limit the amount of shiny's being obtained in the game, and you will need to be skilfull in battling with a rare.
Hence why the "The Shady Thief" quest should be the base of this quest to ensure the battler is well experienced in battling against a sharman and at a decent level to visit the island

Due to the time difference and servers it would be hard to battle one another with your catches.
Would like it to be an all out battle (Tounament), but to time consuming and time difference will matter to most Midnight here afernoon there.