As a lover of words, I hate hearing media airheads repeat heresay word and slang origins that they picked up from some other news outlet, who picked it up from some other news outlet...etc. You can google "pork barrel slaves" and get hundreds of hits with the same explanation, and not one of them provides any references to printed works that would verify it.
According to Oxford English dictionary, the first use of the term "pork barrel" to describe regional favoritism in government spending was in 1909. Since this was a generation after the end of slavery, it is unlikely that it refers to plantation practices.
Also, it's not difficult to find factual accounts of the conditions imposed on slaves. A field laborer could expect a piece of actual pork, maybe, on Christmas. The rest of the time it was hominy and lard, and other cheap, unnutritious foods. This hardly is a good analogy for a government official pouring federal tax money into his own area, which is what "pork barrel" means.
One online source that I really like because they say "probably" when they can't find an exact source:
http://www.etymonline.com