I don't have the answer but have you tried the cupcake forum? Here's the link:
http://www.cakecentral.com/cake-decorating-forum-33.html
You will probably get a lot more help there. Good luck! 
Are you talking about a giant cupcake? Or what kind of cupcake cake? Sorry.
I have made several cupcake cakes. I use a quick icer to do mine and just ice straight across, the icing doesn't fall down in the little open spaces. I do try to fit the cupcakes as close together as possible. After putting a thin icing across the entire top I then build up for the decorating part. I have a couple of pics of the placement and the finished cake.
http://cakecentral.com/modules.php?name=gallery&file=displayimage&pid=977613
I have made several cupcake cakes. I first spread a thin amount of buttercream directly to the board. This will keep cupcakes from sliding around. I then use my large swirl border tip and cover each cupcake with buttercream. I then spread it out very gently and run my spatula around the edges of the outer cupcakes to take off the excess icing. Using the icing tip works great too! Then just decorate like a regular cake.
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