Comet and a stiff Nylon Brush always did it for me, but you've already tried that. The only other thing I used once in while, for really nasty spots, was medium to fine Steel Wool. Watch out for household cleaners as they may react with rubber; check the label, it may indicate if safe for automotive applications.
I wonder if Coker Tire or other Reproduction or Current Tire Manufacturers may be of help. Go on-line or call them, hopefully they'll have some suggestions.
All the best.
Ray