This is a good question, I am also curious to see how other people bend setback handles since I'm sure there are better ways than what I am doing.
For what it's worth, I am working on three bows with setback handles. All my bends are very mild because I'm just learning the techniques. All three of my bows are board bows, 2 red oak and one maple. I use steam.
There are two basic ways I go about it (this is just for the setback handle mind you):
1) I steam the middle of the board then bend it over a form so that immediately out of the handle the limbs bend into straight reflex. After this bend, the board looks like a shallow 'V'. Next, measure several inches out from the handle on each side and bend the limbs back towards the belly (deflex).
2) I place the board into a crude form I made that has blocks of wood supporting the tops and bottoms of the limbs but the middle section is unsupported. I use a clamp to bend the handle down into this middle section. The form prevents the limbs from moving up while the handle is being clamped down.
Sorry, it probably makes more sense with pictures than words, so if you want I can take some photos.