Well the rain has gone, so here are more photos:
Trailer from side
Additional reinforcing gusset in corner to take extra weight when parked on its end
Joint ready to fold, with inner bolts removed, and outer bolts slackened off.
Trailer parked out of the way.
I think this system is not really suitable for folding on a daily basis, as its not fast enough, but its fine for winter. I would also like to improve the joint in some way, but a box section simply can't hinge, and a channel wont handle the hinge bolt in the centre of the bar.
I could probably form a small box section by welding a plate top and bottom of the two flat sections in the space between the two sections of draw bar, and that would add some torsional strength, but it's not a big change. Currently the rigidity depends on the bolts being tight. However if they are loose, the whole thing is still secure, just it will flex. And if you towed it any distance like that, it would likely start to shear the bolts. With a plates welded in, it would likely flex far less when loose.
I am also thinking of building a side loader, as its pretty impossible to slide the AI on or off with those kayak mounts from the back, they stick to the hulls too much.