But, I think it is folly to assume that mere distance or novelty will imply lack of control. If seasteading is successful in its physical engineering efforts, it is more likely that law will change to assert governmental control than that seasteads will be allowed independence. Ultimately technology is not a fix for bad policy--- it provides just a brief window while policy-makers catch up. Politicians and judges are humans, not machines, and they are as fully capable of discerning intent and adapting to change as any. (Hackers are capable of inventing all sorts of reasons why bits are just bits, but jurists and copyright lawyers know better.)
(I think the Seasteading Institute FAQ on this question completely misses the point. It's not about "what will the governments of the world put up with" but "how long will it be until you're classified as belonging to some existing state.")
On the other hand, there is an encouraging trend towards regional autonomy in many parts of the world. This is something I think might be more promising for libertarians to work towards. If secession is good for the Balkans, it should be good enough for the U.S.? (Well, maybe that argument needs some work.) Right now St. Bonifacius needs the permission of Hennepin County in order to leave it. But the moral calculus should tilt the other way--- Hennepin County ought to have the consent of St. Bonifacius in order to govern the city. Moreover, this is a political change which can be done incrementally. A state, a county, or a city could change its laws to permit secession without requiring a top-to-bottom change in how government works. But the end-point would be a complete reversal of the top-down way local government is currently structured.
As a practical matter, it probably makes sense to require some degree of contiguousness, and a threshold of political coalition-building. A city on the border of a county might be allowed to switch, but not one in the middle. A lone homeowner might not be able to leave a city unless he or she convinced some suitable neighborhood to break away as well. A county might switch states but not become a new state all by itself.