I've started to put together some numbers.

With a foot long top tube and a foot long bottom tube (weighing in at roughly 2.2kg and 5kg, respectively) and 1.5kg of aluminium struts, the center of mass of the 5'8" OTA is approximately 2 feet up from the back end. I guess this is "normal" as I know folk weight the back ends of ultralight dobs

I did some rough calculations myself first and checked this using Robert Royce's spreadsheet for the 3-section open newt. I came within 1.5 inches of the value given for a 0-length center tube. This is all just on paper at the moment.

Looking at Toshimi Taki's 8' newt, he's about that from the back. Though materials/etc are different, he weights in at 7.8kg for f/6, I'm around 8.7kg for f/7.6. The Explorer 200P f/5 is 10kg/8kg with rings and dovetail on/off, the Skyliner 200P f/6 is about 11kg. This makes my 8.7kg seem reasonable - it passes the sanity check, anyway.

If I'm sticking with a single truss design, I may therefore go for a 2'6" bottom tube and a 1' top tube, something a little more like the proportions of the Skywatcher open truss, or the Meade Lightbridge.

However, this raises two points in my mind. Firstly, the key benefit of the Serrurier design is that, if I have understood correctly, any sag experienced front and rear will not greatly affect the collimation, as it can with a single truss design. Secondly, if I'm going to be hanging several kilos off my tube, I'd best be confident it's all going to hold together.

I will continue to consider whether I will choose a single or double truss design - I can still bulk buy the aluminium broom handles at £1.40-odd each. But as I'm planning to construct the tubes with hardboard and laminate stuck together with a £1 tube of "high strength" adhesive, I could do a lot worse than to add outer rings and coat the assembled parts in polyester resin before loading them.