Just out of curiosity, why do you want to use a substitute? Horn Knocks aren't that expensive, and fairly easy to install. Then your warbow would be closer to authentic. The only real substitute for horn I can think of would be plastic, if anyone made faux knocks. I haven't seen any.
If you are going to try wood, I'd use something realy, really hard. Lignum Vitae, Verawood and Snakewood are all incredibly hard, with a Janka Hardness rating of 4500, and 3800 for Snakewood. Ebony is around 3220, and India Rosewood (good luck finding some...) is 3170. After that, you get into the more common hardwoods such as Bubinga, Osage, Hickory, Brazillian Cherry, Purpleheart, etc.... The good part is that when polished, all of the really hard woods are exotically beautiful.
I've made a few warbows with self-knocks, and they work fine. I don't know what weight you are aiming for (sorry, couldn't resist....), but I've made a self-knocked warbow up to 90 pounds, from Red Oak, and I haven't had any issues with it. I just use a regular Flemish string, though. If you are going to use a Fast Flight string, you might want to re-enforce the tips a bit.