I have heard (read? where?) that Jimmy crack corn, and I don't care! refers to making corn alcohol!

which makes perfect sense! the masters gone away, and someone has started to ferment some corn for a beer.. (unlikely there would be time or the ability to distill it into whiskey)

and that crackers also refers to small farmers who would crack or ferment their own corn.

crack is also use in processing petrolium.. and for processing pine tar into turpintine..(and some say it's for cracking turpintine that crackers are named-)
but i agree that oh crackey is likely to be a euphonism.