Hollow dummies? that's a new one
I would normally not dowel dummies, but how sturdy are they? The top ones I would just stack, Im assuming they are sturdy enough to take the weight of the ones above? The real cake I will definitely dowel, and the bottom dummy as well if its not very sturdy. If the dummy touches the cake I will add a board, if it touches another dummy I will not bother.
I don't do BC cakes, so cant help you there.