Dang, and I thought I came up with the idea. I would say you do just a pumpkin size batch of in-the-shell fermentation and put the rest in the carboy until the 'primary' pumpkin batch is ready in a few days. If you pour the wort into the punkin just after flame out when it's really hot and put the 'lid' on it, it should sanitize the innerds and the loose-fitting lid should allow CO2 to burp out. That's the easy part. My question is "would it really make a difference in flavor?" If you put a few chunks of fresh pumpkin in the primary, that would do the same thing, right?
I would think the really cool thing would be to find a 5-gallon monster pumpkin and 'cask' condition the brew. Put a weldless valve in it and serve from the pumpkin at a halloween party. A little dry ice through the lid hole would add to the effect and chill it too. That's the kind of crazy fun I signed-up for when I started brewing.