For the record, I think you could make an upper if you reinforced/thickened it sufficiently. In the AR design the aluminum upper and lower halves actually play no part in containing the cartridge during firing. The locking lugs are part of the barrel extension which is more or less permanently screwed onto the barrel. The bolt itself locks into this barrel extension. The only stresses the upper and lower halves deal with are transferring recoil energy, and keeping all the mechanical parts in their correct places during cycling.
Still, and while this isn't a true total plastic design, I think taking bits of small diameter steel rod that you can get at the hardware store, and carefully integrating them into the plastic part as it's being built up by the printer might be a good stop-gap until we get good plastics that can handle the job on their own. Heck, most of the "polymer" parts you have in the gunsmithing industry currently are fiberglass reinforced.
As for making a Liberator Mk-II, a ball and cap pepperbox might be the way to go to start with. Another route I was thinking was essentially a gyrojet type design. Small rocket propelled projectiles, self-spinning, and the only purpose the plastic gun serves is to provide an initial firing guide (the plastic barrel), a feeding mechanism, and the initial mechanical energy to ignite the cartridge (the hammer).
Hey Mods, since this discussion has ventured away from the politics and into the design, do you think we can get it moved to the Round Table for continued discussion during the Christmas break?