The two properties that make a wood good for a belly slat, are elasticity and compression strength.
Elasticity is a technical term for stiffness. We want stiff wood for a belly. If a wood isn't real stiff, then it takes a greater thickness to make a given poundage. Thicker means more stress on the belly side. So your denser, stiffer woods work better against bamboo which is also very stiff. You don't think of boo as being stiff, but it is. It doesn't stretch when bent, which is why the belly is put under more compression stress when you use a boo backing rather than say hickory. Just about everything inside a boo belly slat, is under compression.
Compression strength is the amount of stress a wood can take before failing. This is a little more complicated than it might seem. You can have a wood that will bend a long way before breaking, but it might not be stiff and therefore isn't being stressed as much for a given degree of bend. These are usually lower density woods. Or you can have a high denstiy wood that is under more stress per degree of bend, but if it is brittle then it will fail under less stress than a less brittle wood.
I think you want stiffness and low brittleness in your belly wood. You have observed crabapple is dense, so that is part of the equation. Whether it is brittle or not, you'll find out when you hurry up make a backed bow.
