I always wonder what the dogs think about lugging a pack. But they never seem to mind. Dogs love having a job, and if the job is to carry their food, so much the better!
Lot of good dog packs out there. I've always found RuffWear stuff to be really good. Their Approach Pack ($75) is a very functional dog pack. Comes in five sizes so you can be assured of a good fit. It has a very comfortable suspension, a design that puts the load forward, and nice touches like reflective trim and waterproof zippers. For the well-equipped dog on the go, it's the ticket.
REI makes several dog packs, including the REI Classic Dog Pack ($55 in medium). It's a simple saddlebag design, but has nice touches such as a short integrated leash and a handy grab handle in case you need to fish your dog out of a stream or help him/her over a rock. Plenty of room for doggy food and toys.
Or take a look at the Granite Gear Long Howl pack ($100). Again, saddlebag design, five sizes. The bags detach from the harness, and the harness can then double as a light coat, which could be handy.
You're right about the 20 to 25 percent rule for dogs. If the dog is new to carrying gear, start with just the pack, maybe on one of your daily walks, and then add a little bit of weight. And of course, make sure you have the right supplies. Dogs need leashes, some sort of pad or ground cover, toys, snacks, collapsible food, and water dishes. Make sure your dog gets plenty of water during the hike. Remember, dogs can't sweat, so they have to pant to cool themselves. They can overheat pretty easily!