Johnfitz, there are many ways to use the word "crack," some probably not yet in dictionaries. Your definition of "crack," having to do with gathering corn (like the explanation I prefer, cracking dried corn for use) seems to me to have more validity than those that are based on drinking corn liquor.
However, we shall never be sure. Richie has suggested snoring (see way up above); telling stories and jokes ("cracking corn") is another possibility. Or was it just a noise to add jest to a minstrel routine?   
You may as well stick with your interpretation.