Well getting design paralysis seems there is no room under the seat for much hardware and little room for getting the tilt mechanism [ as described by Danny C ] and a adjustment mechanism ...
Remembered I had some heavy duty hinges 100mm x 100mm x 3mm so thin and big enough to be welded straight to the seat frame and thick enough to take some 8mm clamp/adjustment bolts ?
So do they fit ?
Well yes they will , only concern is they will be welded to some 1.25mm x 1.5mm box ? so may need a little reinforcement ? however it will be sat on the main frame so spreading the load directly to the frame and no just the hinge ?
Other plus sign is that I can clean and weld this onto the existing seat frame without disturbing the existing mounts.
Then I can ride with the seat clamped to the frame through the hinge and make sure that works as planned.
I will probably trim the front tab down to 1/2" below the frame so it helps locate the seat laterally , then I just need to get the rear mount easier to undo without it distorting around the frame and then not being able to lift free.
Any thoughts before I commit ?