Justin,
You ask, rhetorically, whether the "slaves didn't have a right to freedom even though their slavery was legal?"
The simple answer is "No". Certainly, there was no "natural right" because such things don't exist. The right existed within the logic of American law, but the slaves didn't have it. Rights contrary to the condition of slavery and indentured servitude had been recognized, just not applied universally. Another way to look at it is that slaves were considered to be outside the covenant of the Constitution altogether and indentured servants were more like foreigners than citizens for the duration of their service.
You try to prove that property rights are ahistorical because monarchs didn't extend property rights to the slave classes, except as custom. Okay, the slave classes WERE property, so you're wrong. You can't show me a society that hasn't recognized property rights and there never will be one. The question is the extent, character, and logic of property rights.
To get back to the questions arising from the "right to food" debate, it seems clear to me that under any socialist legal ideology the one thing a worker could be guaranteed rights to would be the product of his own work. That right is one that "positive rights" inevitably violate, except in the case we've agreed on.