In case anyone has this problem in the future, I chose to go with the duct tape and bleach route. Wrapped 3 or 4 times around the top, pressed it down nice and tight, and made a 1:4 solution bleach to warm water, enough to cover the tops of the posts. After about 30 minutes, I dragged the kegs out of the walk-in, removed the tape, and hosed them down really well. Just about all the mold came off, any stubbon deposits I hit with a toothbrush. And the beer survived just fine
