This is nice in that it has standardized threaded mounting points, but doesn't solve the distance from the chair arms outs to the keyboard halves. So there'd still be some doing.
The cable connecting the two halves of my ZSA is just a standard 3.5mm TRRS audio cable. Moonlander docs say max length is 6ft~=2m. That should be plenty to route it around the back of your chair.
sure, that's not the issue. The issue is that these mounts let you put a moonlander on tripod bolts. They do not address attaching the so-equipped keyboard halves to the arms of a chair.