-- Last Updated: Mar-20-13 3:37 PM EST --
Two of my canoes have tie-downs for packs, but the method for attaching them to the hull is different so I'll leave that part out. I'm actually pretty certain that the area in the chine of of the hull (where the greatest curvature is) will be a good, strong location. Coincidentally, in most parts of the boat that will also be right along the edge of your foam core.
I use four tie-downs per pack location, in a square pattern. I use rope, looped over the pack in the form of a diamond-hitch. It's secure, tightens with sort of a "pulley" action, and the packs stay in place pretty well. With four attachment points contributing, the tightness of the rope at each attachment point isn't all that great (shall we call it "moderate"?). It need not be as tight as you can make it - just "sort of snug" works pretty well. Don't forget that your packs will float, and in a swamped canoe they won't create nearly as much force on the tie-downs as would be the case if you were rolling the boat back and forth on dry land. The canoe, which is very light, will be "the rag doll shaken by the dog", with your packs being the dog. In other words, the packs will be the major mass you are handling, and the canoe will just be along for the ride, as far as weight and stress goes. It won't be near as bad as you expect.
If you want, you could avoid anchoring your bag straps over the foam core. You could use two anchor points, on opposite sides of the boat, and have the strap split into an inverted "Y" where it comes off the end of the bag. The split "Y" could be made of rope more easily than strapping material. As I mentioned above, I don't think your float-bag straps will see that much stress. I'd probably use two anchor points (not over the foam core) just to be safe, but that might be overkill. Even providing two tie-downs ON the foam core would cut the stress at that location by roughly half.