I found good notes online about building a tarp out of silvered LDPE and using strategically placed "transparent duct tape" to carry the loads. At least one experienced camper has built several of these and used them for years, and I thought if it works for a tarp, why not a pack, except I wouldn't need the heat reflective stuff for a pack.
That may have been my videos
you found. LDPE or polycryo would not work for a pack. Silnylon or cuben would be the lightest fabrics you could use for that application.
Re: Pumpsack - cheating at what? It's not really needed unless your pad has down fill but may make inflating easier on you.