First of all, I love all of the tutorials and tips on making topsy turvy cakes!! Thanks!!!
I have to make a 3 tiered one with a dummy on the bottom. So now I'm confused b/c I won't be able to cut out the top of the cake on the bottom tier. So how should I put them together? My first thought is to just dowel the heck out of it but of course I'm worried about stability.
And I won't be able to assemble on site, any suggestions????

You could cut out the circle all the way through the dummy, trim it to the level needed, then push it back in the hole with a bit of glue.

That sounds a little difficult!?
Is there anyway that I can still just stack on top of the dummy??

I don't know if this helps, but I made a topsy-turvy cake for my daughters second birthday. I made a large cake on the bottom, that I didn't cut at an angle and then I bought two precut Styrofoam dummies, covered them in fondant and decorated them as I would any other cake. When we were ready to cut, I took the dummy cakes right off. I put dowels in it to hold it into place. I don't know how to add a pic. I would if I could, but it's on this site, I posted it.