The backpack alone will not hide the smell. The waterproof aspect when it comes to the zippers are just little flaps OVER the zip so water doesnt get to it. The smell on the other hand is free to pass through the zipper.

Your best bet is to get a jar that is airtight. Even then it wont protect you from someone searching your bag. Believe me they will open your metamucil jar quickly.

You definitely do not want to swallow that shit with a string... could do harm to your body which isnt worth it. Just grow your own!