How big is the barrel? If it's not too big, as in 15 gallons or less and you have a good sized pot, I'd consider filling it with boiling water then seal it up with a wooden or rubber bung. Empty, repeat, and cross your fingers. Word of caution though, this will strip out some of the barrel flavor and move you a little closer to a neutral barrel.
I pulled the bung out a few months ago and it looked like mold on the bottom of the bung. I had it soaking for a week or so with the same water without any potassium metabisulfite or Citric acid.