Securing the dummy to a solid, weighted cake board should eliminate the top-heavy issue. I think it will be much more difficult to ice the cake with a dummy on top as it will want to move around. And to secure it with the center dowel method would likely require two dowels, lest the dummy might spin on just one. It will also be a little trickier to cut and serve.
Fishabel, excellent advice regarding covering a barrel shaped cake. They can be tricky!