K, a good quality epoxy (all are not created equal, check hobby stores) will do the trick. If it's a big crack, art shops usually carry black pigment, and you can mix a little (experiment first) in the epoxy to make it blend. I would suggest fixing the crack before applying the hooflex, or at least not getting the lube in the crack before fixing it.
I suppose one could debate which to do first, but it's Fords and Chevys, and there may be an answer in theory only.
I had just lubed my horn handles last week, and had thought to post a winter advisory, so I'm glad you did. Wood used to be my preference until somebody who really knows knives (he's doing the WW3 handles) gave me a keen grasp of the obvious: everything organic eventually cracks (or breaks down somehow). For example, ivory is beautiful, but a pain to maintain.
Hope it goes well....Dan