Dean Torges used steam on green osage. He thought it would also force some of the moisture out of the wood. If you use moist heat, steam or boil, be sure to seal the back first with shellac. Shellac, unlike other sealers will withstand the heat and moisture and still protect the back.
I use shellac on the back of any bow I make, seasoned or not. Too many times I've exposed a seasoned osage back just to have it check.