CiloGear Dyneema 30L WorkSack
Even at less than one pound, the CiloGear V4 Dyneema 30L WorkSack is the most durable summit pack of its kind. The Dyneema 30L will still be in prime condition long after any other 400 gram 30L packs. Kelly Cordes calls it "My favorite pack ever."
It has the same lid design as the normal V4 30L. It shares the hipbelt of the regular 30L WorkSack, or you could fashion something as light as just a BR Belt to hold it tight while climbing.
In the words of Fabrizio Zangrelli, The Dyneema 30L WorkSack is the summitpack. That's coming from a guy who used to make climbing gear for a living.