Thanks all for the comments.
David- The poles may work- they certainly are worth a shot for the price you paid. I have bent a few ski poles taking falls on the slopes, so I think there may be a fair amount of variation between the quality of the aluminum. But I would be curious to see your results. I do like a good repurposing project!
Kevin- You are right about the high cost of carbon versus aluminum. Depending on the thickness of the aluminum, you may find a good balance between wall thickness/strength/rigidity/weight. I too thought about a metal ring to in effect replace the zip tie. One issue I ran into was that the ring wanted to slide down the legs, and getting a good solid grip between the legs and the tube was difficult. Even when the zip tie was strong enough (before it had very many 'actuations'), I had issues with slippage. That said, there's alway more than one way to do it, so Id love to hear your results. Lastly, if you go carbon (and maybe aluminum for that matter), you may consider filling the tubing in the area of your joint to support the tubing, and avoid crushing due to the stainless tubing edges (hope thats clear).
Doug- I haven't had issues with the legs sliding in the current permutation. I think this success comes from having a really tight lashing, and extra grip from the rubber. I at first made plans to add cord to the bottom, but later found it unnecessary.
Jolly Green- Most of the time I am above tree line, so it would be tough for me to pull this one off often.
Nick- thanks for posting the photos of your stool. I had seen pictures of this stool before, and it was what I based my initial prototypes off of. Once I figured out the correct angles of the rivets (which required some head scratching), I made a great looking tripod with 2 rivets and some carbon tubes. However, this proto didn't pass the sit test. One good sitting, and I heard cracking. When I got up, my beautiful carbon tripod was in pieces. I had even filled the tubing with smaller diameter tubing to help take some of the pressure, but it did not work.
David- You are right that a good fit to the triangle up top is critical. Years of sewing backpacks, and a degree in apparel design helped to get this part of the equation dialed in fairly quickly. I am not sure that the height of the lashing has too much effect, but the quality/tightness has a big effect.
Hope this helps!