I have tried dozens of methods for tieing everything together so the spars don't slip out of the fittings, including, like you guessed, running cords through the spars and fittings(like shock corded tent poles). This system, however, prevents me from plugging the ends of the spars with small pieces of carbon fiber rod. I like to plug and glue the ends of the spars to prevent breakage at the spar ends.
Soooo, my current preferred method (for the vertical spars) is to connect the waist belt to the pack bag with overlapping omni velcro that runs through a loop on the waist belt and then back upon itself. It is secure and adjustable and is working very well. I just returned from a 4 day backpacking trip where I was carrying 30+ pounds of gear, going up and down ropes, sliding through mud, crawling over rocks, etc. and had no problems with the spars coming out of the nylon fittings. My wife was carrying about 25 pounds. My wife and I both used packs I made with the lightweight carbon fiber frame. Hers weighs about 10 ounces and mine weighs about 15 ounces (pack+frame+waist belt+front bag or shoulder straps).
The top (horizontal)spar can be kept in place with a piece of cord running outside the spar and tied to the two top fittings. I'm currently using an aluminum top bar (see previous photos) and that is kept in place by putting a few small dents in the aluminum tubing ends. I use a nail to do this. I place the small dents in the aluminum tubing ends so they can't pull away from the barbs on the nylon fittings.
I've designed the pack so it can be taken apart and put back together. This allows me to run the bags through the washer after each trip. I had quite a bit of mud and clay on my pack after this trip, for example. When taken apart the pack fits into a bag about the size of a tent pole bag.