If you have the patience, you can soft-reset to make Mirage Island appear, just like soft-resetting for the Lilycove Department Store. Get a team of six Pokemon, one of which knows Surf, and save your game in front of the old man in Pacifidlog Town. Don't turn on your game again until the following day (wait until you're done playing the game for the day before your save) and then talk to the man. If he sees Mirage Island, you're good to go. If he doesn't see it, soft-reset and try again. The number will continue to be randomized every time you reset your game until you save, at which point you will need to wait another day before you try again. The odds of Mirage Island appearing when you have a full party are about 1 in 10,923. So, you may have to reset a lot before it shows up.