Yes, really. You can justify anything, no matter how vile, using Libertarianism.
http://cog.kent.edu/lib/Philmore1/Philmore1.htm
Really.It seems to be a basic shared prejudice of liberalism that slavery is inherently involuntary, so the issue of genuinely voluntary slavery has received little scrutiny. The perfectly valid liberal argument that involuntary slavery is inherently unjust is thus taken to include voluntary slavery (in which case, the argument, by definition, does not apply). This has resulted in an abridgment of the freedom of contract in modern liberal society.
Since slavery was abolished, human earning power is forbidden by law to be capitalized. A man is not even free to sell himself: he must rent himself at a wage.4
People are only allowed the temporary security afforded by capitalizing a portion of their earning power (i.e., by renting or hiring themselves out for a specified time period), but are denied the freedom of obtaining a maximum of security by selling all of their human capital. The owners of nonhuman capital (e.g., money, machines, buildings, etc.) enjoy the contractual freedom of either hiring out their capital or selling it, but state interference in the marketplace prevents the owners of human capital from exercising the same liberty. And yet the principal difference between selling and only hiring out an entity is that of selling all or only a part of the services provided by the entity (i.e., the tenure of the contract).
