"I'm wondering if the stave and sinew curing together would prevent cracks forming in the belly."
Now I'm not a sinew guy, and I've never done it, but I'm having trouble visualizing why this would help prevent cracks in the belly. Because the green wood accept the increased reflex better ans the sinew dries? Reduced wood dries pretty quickly, and wouldn't reducing alone eliminate this?
It seems to me that if green wood were reduced to near-finished dimensions, sinew-backed and held in a form or something, it would probably be fine. But, I have also found that both drying wood and drying sinew do, or try to do, some unpredictable things like uneven drying, lateral warping or twist, reflexing more on one side, or one limb, more than the other, etc..