Re: BioShock 2 - Little Sister
I was thinking about doing that myself. It is not that extremely heavy, it would all depend on how much weight you would handle yourself on your back.
The low-res game model that people use for the Pepakura helmet was made in one piece with the backpack plate, probably just to save polygons, but really the backpack plate with the rack for the little sister is not a part of the helmet, so having a little sister on your back would not drag down on your helmet if you made it that way.