Have you given any thought (is it even possible) to making midspan devices? Looking for something that will allow in-line tapping of the duct rather than needing to place a T junction and an end point to reduce the required footprint. I suppose if you were to make it, someone would want both directions, but I'm only really looking for the outtake version. The idea was spawned by duct connections being on opposite sides of each other, but I wouldn't mind the corner version too (ducts on adjacent sides.)

I use underground duct -> end-point outtake -> end-point intake -> underground duct designs a lot, with smaller pipes coming out on the sides.
It looks ugly and is clunky, but it does the job, so I second the idea of a mid-point intake/outtake that you can connect UD -> MP I/O -> UD.