Personally, I would prefer the incline step for this, rather than the ball. I would be worried about losing my balance and falling with my pregnancy belly to the floor. Balance isn't always what you want it to be in pregnancy.
I am wondering if, in using the incline step, this would become more of a chest exercise? I think it would still target the back principally, especially if you make enormous effort to contract those muscles as you bring the weight back over the head towards the chest, leading with the elbows, but I think it might bring the chest into play more also.
Interesting.
Bringing the chest into it also may not be a bad thing, after all "this is an upper body tape, right?" as Cathe says on PUB.
If you are also targetting the back with dumbell rows and barbell rows as on CTX back segment, I would not worry about it too much.
Things have to change somewhat in middle to late pregnancy, 'aint no gettin' away from that!
Clare