I really didn't lose much wort when mashing canned pumpkin when I cloned Pumking. It's definitely a sticky mash, requiring rice hulls in my opinion. It's not like the pumpkin is dry...there's definitely water content in pumpkin, so I don't see why it would soak up that much wort. Even if it did, you can just increase your volume to get 5G into the keg/bottling bucket.
Honestly, pumpkin ales are mostly about the spices...pumpkin doesn't have a whole lot of flavor. I'd feel weird calling it a pumpkin ale without using pumpkin, so I used Libby's canned.