You could have barrels overbored if walls are thick enough and also cut long forcing cones.
+1. Some honing could almost certainly be done to reduce barrel weight.
Mike, I'm thinking a styrofoam stock would require very light recoiling shells.:)
I expect you might be able, if starting with a very light Fox 20 with 26" barrels, to get it under 5#. But I'm thinking you probably would not like the results. It'd be hard to give it good balance (easier to take out weight behind the hinge vs in front) and most people don't shoot guns that light very well.
