david, i like your attitude, in wanting to experiment. and i agree with marc: usehickory overtop, should work just fine. I've noticed that on bows up to 60# at least, the grain on the hickory can be damn wild and still be perfect for use...in fact the only time i've had the hickory fail on me was an edge-ringed piece with perfect parallels from tip to tip. Will you be using a thick piece of osage, or laminations? If you use laminations, you can pretiller it somewhat by gluing it up in a jig.
And someone mentioned bamboo. I say give'r. My bamboo backed bows are mega strong, and if they are a bit touchy to tiller under the nodes, they are worth it. One bow is only 1 inch wide and still draws 70#, at 62" length. An english warbow, say, 72 or 78 inches long, backed with bamboo, should safely draw over a hundred pounds no problem.