I ended up brazing my grate. I'm doing a lot of renovating here and don't have my stick welder hooked up yet. I refreshed my knowledge of cast iron repair, and being the nincompoop that I am, ignored most of it and just went for it. The proper: Cast iron cracks should be ground out in a "U" and then brushed with a stainless steel wire brush to remove all possible grinding debris. There is carbon in cast iron and it will interfere with welding or brazing. Then the piece should be carefully and slowly preheated plenty hot. Stick welding can be done with nickle rod, and an inch at a time is suggested, with a good peening with the point of your welding hammer to relieve stress. The piece should be immediately put in a bed of dry sand or lime and covered completely for a long, slow cooldown! I've heard brazing is best done with bare rod and special agressive brazing flux on the clean metal.
Well good ole Dan just went for it late last night and the braze "took" pretty good. Neutral oxy/acetylene flame and coated brass rod. I just ground the cracks a little and started brazing. I set the completed grate between two slices of 3-1/2" fiberglass insulation to try to slow down the cooling! I'm hoping this repair strengthens the grate enough to prevent it from cracking all the way in two. It had that funny feel like a dull tuning fork with the crack starting to widen after a few coal fires. At least now it's held together. I made no attempt to pull the crack together, the grate was very slightly warped and offset. I think the whole piece would have to have been red hot and malleable for me to try to pull it together. It's a sloppy braze, I might go over it again. Got lucky and will see if it holds.