Not too impossible, I mean, you can make it like the inventory systems. Simply just move it to a boundary point, create a trackable of a custom model (the size you want your button or something) then register it on click.
Requires JASS though, and it isn't really "simple". I am hoping to make a mod for this. I am working on it and it will hopefully be like that but an ability menu for Darkholme's Dungeon Keeper.