If you're packing things into one-pound bundles (to make up that 50-pound load of food you mentioned in another post? smile ) why not just leave it in the plastic jar? For the amount of food you're taking, you'll never notice the 3 or 4 ounces of jar versus one or two ounces of bag you'll be carrying. Besides, one plastic jar (which can be reused for milk you buy in a box, or other non-milk stuff) would seem to be more environmentally friendly than all the bags you'll use up instead. Also, it's going to be a lot harder to poke a hole in the jar than in those multiple ziploc bags, especially considering how often you'll be getting in and out of them. (And, as the powder sticks in the closure, as you know it will, you could end up with an unsealable bag full of powder; I doubt you could easily wreck the threads on the jar.

For another method, see Colin Fletcher's Complete Walker (any of the four editions) where he describes using a "squirt" dispenser (like a mustard or ketchup container you find in some restaurants) to hold things like milk. It's in the Kitchen chapter.