Ideas enough on this forum, that's for sure!
I'd be more inclined to treat the tube under the seat as a nice anchor for tie-downs. I have an Airhawk seat cushion and would rather run a strap under there than under the seat (because the seat is harder to detach than a normal motorcycle's, and don't want to interfere with the wires and fuse blocks there).
One concern with putting things there is weather/moisture exposure. So, I tend to drop extras in my topcase which is very easy to keep dry.
The rear swingarm tube might be good for a trailer mount, conceptually. Maaaybe the under-seat tube could partly anchor a fairing.
I don't know about mounting a trailer on that part, it would get some significant shocks I think.
Anyway, I did a quick mockup(not scaled correctly) in a 3D model:
So far only a screenshot, I have to wait 24 hours before it can be published at Thingiverse.
The idea is to have two parts which you can
snap fit together. This way it is mounted sturdy but using a screw driver you can uncouple them later if you want.