Ouch. I am by no means an expert on brushes themselves. I'm only doing my first knot transplant myself. BUT, I would guess that if you totally removed the knot, you could get a yellowish epoxy and try to essentially glue it back together. If you were careful about it, you might be able to get it done without removing the knot. Put epoxy in, clamp it together, let it dry, and viola (hopefully)
