how is the middle of a folded Brompton supported?
From a phyics or engineering point of view, I don't know, something on the order of 20/30 kgs, maybe?
From the warranty and manufacturer recommendation, the answer is 0.
A folded Brompton (and other foldable bicycles) are not supposed nor meant to carry any weight.
You can still revert the order on your trailer, put the heavy stuff on the bottom (which is  in any case a meaningful thing to do) and then the Brompton on top, fixed with some bungee rope or with some other kind of band/belt.