I quite like the idea, but perhaps some shaping of the pads - maybe one side shaped to X-busses, and the other to Y busses...
Also some way to incorporate SMD pitched components on main board - or daughter 'conversion' boards with solder mask - so they can be dropped flush to the carrier 'buss' board.
My mind is racing, and I can see a few ways to make this work really nicely.
Image is just one part of the idea I'm suggesting... works top & bottom copper, with rotated 90-degrees for other side of the board as mentioned.