Unless you're carrying enough in the pack to keep the bladder in position you'll have to sewn in a panel to keep it from just sloshing around in the main compartment. I don't know what sort of padding the book bag has, but if it's just canvas of some sort you'll be putting the bladder nearly directly against your back, that gets pretty hot and sweaty.
I've just been putting my camelbak bladder in a small kitchen trash bag and then packing soft materials around it. I've been concerned about it shifting around in my pack and being punctured, hence the idea of making a pocket for it. My back does get a little warm with my current daypack and I will eventually replace it - I'm just not ready to buy/make a dedicated daypack.
4. Don't worry about a pouch. Just cut a small hole in the top of the pack for the hose, and put the bladder inside.
I like the idea of the insulated pouch using closed cell foam. I wonder how cool it would keep the water...
My backpack doesn't even have a form fitting pocket for my Camelbak bladder but I looped a nylon shoelace around the bladder opening and through a loop at the back/top of my pack.
I like the idea of using a shoelace to keep it near the top of the pack.
I appreciate the comments that I've received so far. I think I'm going to end up making some cheap and easy modifications to my pack with the intention of getting a dedicated daypack at some point in the future.