In a buggy your best bet for larger lipo's, if you don't mind the weight, are the larger hyperion vx 6500 but that will make you pretty heavy.. the vx 5500 is a nice normal pack that gets you a little more uptime w/ the same weight.
Ive been in some of those longer 20+ min nitro buggy amains myself and the idea I was toying with was cutting away a portion of the body (over the battery tray) then just making a strap to hold it in place that is basically held by a clip, then just using 1 larger velcro strap to hold the batts in palce it would take no time to remove the 'battery door' and swap out packs. I have been using deans w/o issues but the traxxas, as noted above, are a little bigger and easier to handle in a quick change. This coupled w/ the hyperion 5500mah lipo (or larger) should be a winner iMO.
Very nice setup. I'm still looking for the 20min setup with minimal weight gains. I ran a 10 min main yesterday....won the Amain. Ended up using 3300mah of my 5000mah total. That is a 4S 5000 batt. Weighs 470 grams