As long as the stresses are spread out there is no concern or need for anything central.
Sure, lets assume the stresses are spread out. To where are they spread? What are the contact points?
Until I hit ten posts, I can't add images. Let's say we lay the folded bike on its side and look at it from the frametube/saddle edge so that it looks like a "<", with the left of the "<" being the frame hinge and the top of the "<" being the handlebar side.
The only way that the bike can stay "<" and not become "-" is if there is a contact point (or several) somewhere along the inside the two parts of the "<".
I am asking what that contact point is, for a normal Brompton.
I would have guessed that it is the fork hook near the hub but that is on the wrong side: the hook prevents the handlebar side being pulled outward but it doesn't stop the handlebar side being pushed inward. My next guess would be that the fork blade eventually contacts the chainstay.
I cannot comment on the brake caliper, without understanding details, except for an off the cuff that there is a need to choose one's battles.
The Kinetics fork has a mount for a disc brake caliper on the left arm at the back. This ends up on the inside of the fold. Pushing in from the handlebar side, the contact point for the two halves of the bike is the part of the caliper that butts against the chainring guard. Pressure applied to the handlebar side is going to end up largely being absorbed at this contact point and by nothing else. Most likely, I'd expect a wobbly chainring to result which would be annoying enough, but neither the caliper nor its mount is suposed to take large forces along this axis so I'd worry there could be much worse outcomes. I can work around the problem by providing suitable support. My question is how a normal Brompton avoids the necessity to do this.