Interesting idea.
Years ago I played with backpack weight while training for some army stuff. After trying a few different few different angles - plates, dumbbells, lead, tools, sand, etc - I found the best for me was to make sand bags from play sand, gallon zip locks, contractor bags and duct tape. I make them in 10 & 20# doses and load up as needed. I wrap them in towels or furniture blankets and put them in my pack. They don’t shift around or burn through the nylon like hard weights. Also, if I ever needed to dump weight they are cheap and more or less disposable.