I'm eventually going to make another g4 like pack, but with adaptations for my body and likes.
I've been thinking about putting a hydration sleeve inside, but didn't know what to do about leaks. Putting it on the outside, under the pad, might be a better way to go. It would stay insulated, instead of being heated by the sun.
I know some of the smaller Osprey packs have a very curved framesheet, which leaves only mesh touching the back, and they have the ability to hang a bladder in that arched space also. However, they arch it so much, it cuts into the carrying capacity of the rest of the pack. It looks like, with the right bladder, you could carry upwards of two gallons in the arch space. Which makes me think about using them for Death Valley.
Osprey's larger packs do not have that arch space.
Not sure how to do it on a G4. To help prevent it from leaking into the internal cavity, it would need to be a sleeve between the back of the pack and the mesh holding the pad. Might need to rig some hangers from the shoulder strap attachments.
But how to get to it. The mesh at the top would be in the way big time. You'd need to unload the pack and then bend it in half to get to the top of the sleeve.
If you had a sleeve between the back of the pack and the internal cavity, accessible from either the internal top of the pack, or the external top of the pack, that would help.
Have to do some hard thinking about how to do it externally, due to a lot of the structural enforcement for the shoulder straps are in that same area.
I have been thinking about some full-length mesh on the back that would open up the center, and fold back, and overlap with some velcro or something.
That would make an external sleeve readily accessible.
Don't know yet.