I assume you're backpacking, but is there any climbing, any river crossing or canoeing?
my eldest daughter has just started DOE and I cant believe the weight and size of the stuff in the recommended expedition kit lists. Does anybody really stagger around the countryside with half of millets & blacks on their back? it doesn't match my thoughts on lightweight (with my knadgered old back and fondness for motorcycle camping I've got keen on cutting gear weight down)
I've been wondering how to do this myself, we tend to go on family days out, 3 kids to hang on to and a rucksack of spare clothes/picnic to haul, usually around a foresaken piece of derbyshire or ruined castle in the middle of nowhere, so I've been trying to plan a better camera system for similar circumstances to yours.
Full camera kit rucksacks are a non-starter because for this exercise the camera is the incidental bit and the walking/camping/picnic gear is the main load.
My thoughts are to use a normal padded all weather (rain cover) camera bag, smallest that will fit with the lightest and most flexible lens size you've got (if you're willing to just accept there will be no lens changes). Get it so its belt wearable so you can have it ready to use while wearing rucksack, make sure any shoulder strap is removeable, and a lightweight belt for when not wearing rucksack to stop it swinging around. If you are desperate to take more than one lens then probably get the bag sized to fit the bigger lens, and a separate lens case for the bigger lens, that way you have full flexibility, and the bulk of the unused lens (and its weight) gets safely stashed in the rucksack and not tangled round your waist, or you can stick it on your belt. I've noticed that the Thinktank holster range has expanding cases to suit different lens sizes so may be useful to minimise bulk if you are swapping between long & short lenses, I have not seen any yet though so dont know how well protective they are, especially the expanding bit.
Rucksacks can be water concentrating devices so get a tough vinyl Ortlieb drysack to stuff the camera bag & any lens bags, especially if you're going wading or watersports or run into prolonged welsh mist. Even if its inside a rucksack use the ortlieb. There are lighter drysacks including silnylon but I'd stick with a heavyweight ortlieb type for camera gear. Get one thats rated for immersion, not all of them are. With a bit of air in it it will act as more padding and probably float too. An alternative make to Ortlieb is Exped, they do a range of padded Crush Dry Bags as well, may be worth looking at. Drysacks can be lashed to the outside of rucksacks (eg on top) if you want quick access to camera gear and the terrain is not too rough and likely to damage gear (ie. tuck your camera gear inside the rucksack on scree slopes).
