http://www.camelbak.com/government-military/hydration-packs/mule500.aspx
I am looking for a camelbak that I can wear on top of an interceptor and under an assault pack or large main pack.
Other option is to get a nicer than the issued USMC IBLE Assult pack and put a camelbak bladder in it. Who makes the best assult packs that can be worn in a vehicle? Do any of them hook on to the USMC IBLE main pack like the issued assault pack?
On the same page you linked, there is a flatter-bladder 3L variant. I’ve had good luck with flat-bladder designs for the same reason; I want water on my armor but intensely dislike the way a regular Camelback or similar destabilized any pack I put on there. It’s not something that every truly goes away, but can be attenuated.
It was showing in the lower-left corner of the page you linked: http://www.camelbak.com/government-military/hydration-packs/armorbak
Also, unless something has changed pretty recently, the issue assault pack that comes with the ILBE doesn’t attach directly to it by any means. There is a flap/buckle/strap arrangement on the back of the main pack that can be used for the assault pack, a helmet, an E-tool, etc., so it’s made to be versitile and hold all sorts of stuff of varying size and shape. My Eagle A-III, Arc’teryx Mantis, and Camelback Talon day packs all fit in it just fine.
If you’re still looking for a day pack for the bladder and some cargo that can likely be worn in a vic, chech out the Blue Force Gear DAP pack and the Eagle Yote. They’re both smaller than their 3-day brethren, but may fulfill your need. BFG site seems to be down at tha moment, but…