I first saw SCA and thought, "Society for Creative Anachronisms", the guy wants to ask about titanum chain mail for mock sword fights?
Yes, I know the other SCA and greatly appreciate their trail work and always say so when I hike on by.
Yes, I have used that exact technique - external frame without packbag to move stuff. I agree with you that you are better able to move bulky stuff and of course save the weight and cost of the packbag. I also instructed in its use decades ago when one Boy Scout Troop and one BSA summer camp had packframes for loan but no packbags. In fair weather, they'd toss everything in their sleeping bag. For rain in the tent or a garbage bag. Then use a diamond or spider hitch to secure it to the frame. Here's a link on the diamond hitch:
The Spider Hitch is similar but less formal concept. You start with a 9-12" diameter loop in a long cord with the loop positioned in the center back of your load. Then take the loose end and loop each corner and each side of the frame, returning to pass through the loop each time. Then snug it up.
A few cinch straps are quicker but less versatile (you could use that long line to rig your tarp). Cinch straps are great for single, bulky loads like generators and chainsaws.
Another helpful trick is a lash a plastic kitchen trash can (largest that fits under the sink cabinet) to a frame (especially a "freighter" frame that has a lower shelf for that purpose). Then you can toss chainsaws, oddball stuff or bulk goods (firewood, sand, gravel, deer quarters, geotech fabric, etc), in there quickly and dump it quickly.
For convenience on the trail, secure a fanny pack to the back of the backpack. Or secure the fanny pack backwards on your waist (like in the inner city for concealing a gun) and then you can access repellent, water, snack without stopping. Or put a pocket on your backpack waist belt.