One option might be to create a characterset of the character on top of the island, with the island as part of the sprite. When the character walks onto the island, change it to that, and then move it as you would any other graphic (or allow the player to move it, or whatever your plan might be.)
If you'd like the island to move independently before the player gets on it, just create a characterset of the island itself, and move it as any other characterset.
This might be a little bit sloppy, but it would at least get the job done. If you wanted to make it look a little nicer, you could create additional character graphics for the player "boarding" the island, and so on.
This way, it could all be evented, so you wouldn't need any new scripts.
