I agree, John. I'm contemplating whether to add two supports up from the bottom (like a Mixte frame) or extending the top bar by adding a horizontal tube from the current seat-post top to the old seat-post hole. The Mixte supports could also incorporate a frame-mounted cargo rack that could hold two stealth panniers (to hide a rear hub, or a non-hub drive)
My main goal this week was to find out the most comfortable location for the seat. Now that I have the dimensions, I can begin designing a more professional-looking and stronger permanent arrangement.
I posted this in the General E-bike section because I hadn't seen anything like it, and I wondered why I don't see any examples of rear suspension long-tail (other than oldhaq's a year ago. He started this). I believe this can have a wide application, and many would benefit from it. Very do-able with no welding, and not very expensive. A vice, hacksaw, and a cordless drill at a minimum.
A folding NON-supension long-tail cargo bike with an RC-drive would be a piece of cake. Though I might make the effort to get aluminum for all the frame parts, and I'd use a 20" rear wheel.
I believe this thread has done its job, and I won't post again for a while. Once I have some significant improvements accomplished, I'll start a build thread. Thanks to everyone for your encouragement and support, I appreciate every response.