i've not sewn absorbant layer with batting but i imagine cotton batting would cause major wicking and it breaks down after many washes in the washer? plus wont they be a pain to dry and also more expensive unless they are recycled material?
I have used poly batting (from an old bed pad) in the absorbant layer, and really liked it. The free Ottobre pattern recommends this to decrease drying time. I thought it added softness and flexibility to the soaker. I have thought about using cotton batting b/c it might absorb more, but I am worried it will break down and clump with many washings. I may still try it on one dipe just to see how it goes.
I've used cotton batting before in a soaker and it works great, not as absorbant as say, hemp fleece, but it works just fine. Just make sure you sandwhich it between other layers. It doesn't break down at all. I wouldn't recommend using poly batting as it just adds bulk and no absorbancy. And actually, it's quite cheap to use as soaker material if you get it when it's 50% off at Joanns. Plus the fact that a yard of it is waaaaayyyy wider than a yard of other fabrics. Oh, and also, I've heard it recommended to ONLY use the warm & natural brand, otherwise you may end up with some stinkage.