I tried. God damn I fucking tried.
Mirage Island has a 1/65536 (0.0015%) chance of appearing per day, based on comparing the first two bytes of any Pokemon's personality value in the player's party with the secret number generated once per day. This increases to 6/65536 per day with a full party.
The Pokemon Storage System has 14 boxes, each with a capacity for 30 Pokemon. That's 420 Pokemon in total, plus what's in the party. That brings the total to 420/65536 (0.65%) per day, if you check every single Pokemon. However, you can't run or ride your bike in Pacifidlog Town, where the Mirage Island checker is. You must walk, to and from the Pokemon Center.
The secret number is rolled once the game rolls over midnight, and then is locked in once the game is saved for the first time that day. Which means there's generally three techniques you can do:
>1. Save the game after midnight in the Pokemon Center with 1 Pokemon, locking in the number. Withdraw 5 Pokemon and check them. If Mirage Island doesn't appear, soft reset to warp back to the Pokemon Center, withdraw the next 5 and check again.
>2. Save the game before midnight in the Pokemon Center and do NOT save it again after. Withdraw 5 Pokemon and check, then walk back, deposit them, and withdraw the next 5 and check them. This lets you check all 420+ Pokemon. If Mirage Island isn't found, soft reset to reroll the secret number and do this all over again
>3. Save in front of the old man with 6 Pokemon in your party, before midnight. Once midnight rolls over do NOT save again. Check your current party and if Mirage Island isn't found, soft reset and check again.
I did number 2 for a while before my patience ran thin, then I did number 1. It took two hours a day to check all 420. I did it for two months straight. I got nothing. In the end I got my liechi berries from the shiny Zigzagoon distribution, and then used the Battle Tower cloning glitch to farm them faster than I could via actual farming.
