I think you should be fine with purchasing that even though there is run off. But your best bet would be to back it with something, like bamboo as you said. The stresses of tension would be placed on the boo instead of the sage and the run off shouldn't matter too much.
I'm definitely not the best person to be asking advice, but in my experiences you should be fine with that plan. I mean think about it, you can make a red oak board bow with minimal grain run off, unbacked around 50 lbs. And in my opinion, osage is a far superior wood to red oak. Just a thought.
-Ian