That's off the chart idea. You can't just hold any island out there as all of them will be ruled by next government present nearest to that land. For example if it is in western Pacific Ocean then it is most probably under the rules of USA while the islands in the eastern Pacific Ocean would be under the territory of India. So you go anywhere in the world you will have to face the legislation first to acquire that particular island. It is good idea but bitcoin is not that powerful to get you island as government should accept that particular currency that is bitcoin to let you buy it. It seems to be complex idea and may not be possible at this point.