In reply to James Oswald:
I just went looking for the same thing and ended up buying the Mountain Hardwear Crag Wagon 45 (they also do a 55). Very impressed with it and wouldn't really change a thing. It was the Outside video review that convinced me:
The whole thing feels very beefy and indestructible, including the zips. I always used to be of the same opinion as petegunn above, that extensive zips were a bad idea, but I reckon maybe times have moved on. The zip on my Osprey Mutant has held up really well for years now. That's just for closing the top, but it actually gets a lot more abuse than the Crag Wagon zip when trying to pack it really tightly and then force the zip closed. Whereas with the Crag Wagon, you would mostly load from the top with the zip already closed. So very little reason to be trying to force the zip closed under pressure, which always feels like the thing most likely to damage a zip. Yes, it would be a tricky repair if the zip did fail. But it doesn't feel likely and the convenience of being able to easily access everything in any order is really nice.