Verry interesting, Marc.
I'm curious, should you look here: http://history.nasa....nd/structur.htm, scroll down a bit, under the heading entitled 'B. THIN SHELL STRUCTURES', take a look at the sheet stringer construction illustrated in Fig. 1, do you find it any more practical they've eliminated weight of that inner skin, maybe even more material in using this I-beamesque-construction, possibly fewer welds, and, further, with their running them strictly vertically, taking the compression factor into account they mention later on~wouldn't it do just as well?.. No offense, admittedly, I don't know this first thing about rockets and so feel free to laugh, I just found it curious; your post is very interesting. ...And how much are we experiencing these days, in the way of the buckling, with current design?..
I do find the extrusion process neat, and extruding would-be beams or anything else, such as the corrugated look of your own idea, has a lot to to offer in the way of adding exponential strength, as opposed to sheet metal fab.