Could we buy up enough land to claim it as bitcoin land? Everyone using BTC...
Yes, probably. It would still be a part of whatever country it lies in, subject to its laws. But on a private property, people could do business in BTC, or chickens, or whatever.
Has there ever been a privately owned city? I imagine there would still need to be a municipal government to answer to federal authorities, but the municipal government might have a fairly wide latitude regarding its own laws and policies.