Jesús Huerta de Soto (Wikipedia page) is a Spanish economist who subscribes to the Austrian school of economics. He has made the case for radically freer migration with some keyhole solutions in an article titled A Libertarian Theory of Free Immigration (PDF, 11 pages) in the Journal of Libertarian Studies.

In the paper, de Soto argues that, within the current nation-state framework, migration should be free subject to only the following provisos:

1. Migrants should not have access to the largesse of the welfare state.
2. Migrants should be able to demonstrate that they can support themselves, possibly via a native sponsor (such as an employer).
3. Migrants should not be given immediate access to the vote (though they may be given such access after a reasonably lengthy period of residence).
4. Migrants must at all times satisfy the law, particularly the criminal law, of the country they are residing in, and may be subject to deportation if they fail to do so.

de Soto prefers an anarcho-capitalist framework and argues that the existence of the nation-state is responsible for many of the problems (real or perceived) associated with migration.