"Incidentally, my reasoning for the double top tube is that for a large frame (say 63cm and up), you don't really have a front triangle.... you have a 4 sided shape. The difference between a triangle and a four sided shape is that you can twist the latter. The longer the headtube gets, the more it can twist. By adding the lower top tube, I am thinking that you essentially create the less twisty front triangle that short frames have, but still have the height you need."
The second tube has to help in that regard, assuming that is even the problem. But it doesn't look overly efficient, since while it creates a real triangle one foot of the tube is pretty much at a point of rotation. I would love to experiment with a tube that broke the main triangle up into smaller triangles, but it would be more beneficial for weight than racking in the frame.
I was looking at Zinn's site since he specializes in tall frames. I noticed he doesn't do anything unusual in making his frames, just standard tubes. So larger, stronger tubes seems like they would get the job done by themselves, just deploy the normal improvements for heavy riders since taller is heavier by itself.
If I wanted the frame to be a lot stiffer laterally, I would probably go for much fatter tubes as for aluminum, or some kind of structure a little bit more like a mixte.