I chipped the top of my Rooney brush when i was drilling it out to reknot it, and just glued it back on. It has held together well without any problems. If my chip can hold up, I think your crack will be just fine.
I do not have enough experience to tell you with certainty what will happen, but in my experience, the new knot you put in will expand quite a bit after the first use. The knot will expand with each use and will "shrink" when dry. In my opinion, the crack will get worse (and provide perfect place for bacteria to grow).
Mind you, I have been wrong before.
It should be fine Mark, but I have no experience with boar brushes, don't the hairs actually absorb water? I think that's what was being refered to above about the knot swelling. I have a few old handles with badger knots in the that have cracks like this and larger. But they have been reknotted and covered with epoxy from the inside and I have not had a issue with any of mine. I wouldn't reknot it unless you absolutely had too. Just MO though.