I have done both ways before, and mostly now carry my MSR 4L Dromlite between the frame and bag. Last year, I must not have had the cap cinched down tight on my Dromlite and had a bit of a water problem in my bag. Since than I always carry it out of bag. I just hook the hook handle holder of the Dromlite onto the frame fabric- same way Kurt shows with a Camelback.
When I did carry in bag, I tied P-cord to the 2 loops that hold the Tri-glides and then hung a small carabiner on that cord.