I looked into this last night, actually.
I'm playing the American version of the game, so I'm not sure if the offsets are different for other releases (not sure why they would be), but you are able to edit all 40 basket slots with any item that you want, and use them on the island. I brought a bunch of golden tools and other hold items to give out to strangers at Club Tortimer, and they worked just fine. Just make sure that you reverse the item ID bytes when making your edits. For example, the item ID for the throwing beans item is 307D. It needs to be written out in the RAM dump as 7D 30 in order to show up properly in the game.
Tl;dr - Island basket offsets begin at 0x6E60, and end at 0x6EFF. You have 40 item slots to use, and you have to reverse the Item ID bytes in order for it to work properly. (Example: 307D becomes 7D30)
Hope this information helps!
awesome