It is Rigid, it can support the weight of the entire rig on its back and not deform. And it is certainly just as rigid as some of the scrawny chassis cabs I have seen.
I will cut out the windows. I will cut out the sides. I will use zip ties to mount the body panels.
It will no doubt be rigid, it will not be the same material as a body, it will be structurally complete.
Then where do we stand?
Stand is exactly what we need to do. And by "stand", I mean stand on our bodiless rigs. I know mine would hold my weight, would yours?
What you've done is scratch build a tight fitting body, bolted it on, and called it a bodiless. You've gone one step further than me taking a regular body, bending and tweaking it, bolting it on, and calling it the same.
If you have to use the interpretation of intent as your argument for legality, you've already admitted that you already know that it is not legal.