Well, actually you can toss any bladder on any of the BC's... but you won't gain much. The goal is???
If the goal is to dive doubles with the zeagle, then even though you can put a larger bladder on a zena or a scout, you are sill not going to run doubles with them. For that, you want to be in a ranger... but then the ranger is bulky with the 44 lb bladder - too bulky for travel for me, so I had scott make up my custom "
Zeagle Brigade" which is a ranger harness with the same bladder that is on the Zena or the Escape, or the Scout.
That way, heavy duty, steel d-rings, have the ability to swap out bladders, dive duals, etc... but yet packs up small.