I did this when I was backpacking in Cali. above Camarillo in the hills. We just used a barbaque rack and held it at a decent distance, it cooked great for us, although we did burn the crust a little on the edges.
When I first read this I thought of a very large terra cotta pot. I was thinking "almost a tandoor", and then I realized that's not what you were talking about /img/vbsmilies/smilies/smile.gif Plus, I'd like a picture of anyone taking a really big flowerpot in/on a backpack. /img/vbsmilies/smilies/rollsmile.gif
You can make a good campfire/camp stove pizza with a frying pan and a cast iron lid. That's what I use anyway and it always turns out nice. Basically just stick the cast iron lid on the fire or over a stove till it's really hot, heat the frying pan up then put the pizza in it with the lid on. Cooks nicely in about 5-10 mins.