With osage, dry heat works well for bending and shaping. With some whitewoods steam or boiling works better. I have never worked with slippery elm so I don't know what would woul best.
If you are just adding R/D you should be able to clamp it to a form while you are heating each limb, a little at a time, starting at the fades and working out towards the tips. After it is clamped in the form reheat the wood until it begins to turn color and work out each limb. After you have completed the last heating allow it to cool slowly and also give it a few days to rehydrate before stressing it. This is how I would do it. Whether it will work or not I don't know.
If you have TBBIV, check out Marc St Louis' chapter on "heat treating". Marc uses a lot of elm for bows.