I have thought about this some, but there are problems to carrying it externally (many mentioned in post above mine). An internal frame pack does not have many good spots to carry a bear can. They are heavy and carrying them on top makes for an unstable top heavy pack. Most internal packs don't have much room underneath to strap on a bear can. You definitely don't want it banging into your butt or legs. The classic old external packs have a nice spot but they tend to heavier packs and very stiff (the load doesn't move with you). Strapping it on the side of the pack would be worse for your balance than strapping it on top.

These things are heavy and it is best to get them as close to your backbone as possible. One thing you can do is get the right sized bear can for your trip. I bought a lighter1 Lil' Sammi for weekend trips. I think it is smaller diameter than other IGBC approved canisters. It fits nicely in pack but doesn't work very well for groups or week long trips.