Remember the "kits" they used to sell in shoe stores to make Hush Puppies look new again?
It consisted of a brass bristled brush and a piece of sponge with a sand paper texture on one side.
I've used those kitchen type of wire brushes and fairly coarse (120 grit?) sand paper on the rough side of various things made from leather over the years. It's helped some, and hasn't hurt anything. Depending how how dirty it is, some plain ol' rubbing alcohol before brushin might help loosen up the dirt.
Whatever you decide to try, take some before and after pics. I never thought to do that, and have wished I had on numerous occassions.
Mark