I know a Roadster owner in Hawaii. He told me the Ranger flies out once a year and services all the Roadsters in Hawaii at once. I think they all split the cost of travel. When you split it 4 or 5 ways, it isn't that bad. The problem is if you need something more than a simple service (e.g. you get into an accident or a part needs to be replaced). I don't know what happens then.
It has been over a year, but Tesla has mentioned that Hawaii is a prime market for them because the round trip around Oahu is less than 160 miles. They indicated that they would open a store in Hawaii eventually, but as of yet I see no indication that they are indeed opening a store.