While the torsion bar DOES carry a lot of its load on the outer section of the bar, I don't really think a hollow torsion bar would be feasible.
Consider hollow sway bars- in order to maintain the same spring rate as a solid bar, they have a larger outer diameter. With most folks running 1"+ diameter torsion bars, you don't have a lot to work with with respect to the hex ends of the bars. The largest bars Ma Mopar ever made were 1.25", so realistically your outer diameter would be limited to that, which really doesn't offer much room to expand. Even if you did manage it then, the hollow section would be very small, so any weight savings would be seriously marginal. To go any larger then that you'd either have to neck the bar down (manufacturing nightmare), or replace the hex ends, which would mean replacing your torsion bar anchors and LCA's. Not to mention there's A LOT more stress on a torsion bar than a sway bar- even if someone managed it I don't think I'd run them, gives me the heebie jeebies from an engineering standpoint.
And then there's the fact that folks have issues even running 1" bars. I haven't had any myself, but there are people out there that had header and steering component interference issues with 1" bars.
