Indeed, the nice thing about it is that you could design a big board mix it up with a plethora of different architectures inside them.

A removable back plate would indeed solve most of the watercooling issues with a air-only solution (i understand your air only preference, but with >60% humidity points here on my country, high density only happens with water blocks).

Also, i was thinking about the thermal protection you proposed, it would be nice to have the thermal watchdog in a separate process NOT related to cgminer, as you can see what happened to the S5s with that option.

I was thinking that the boards should go with atx spacers to have them mounted like standard motherboards on the case (but in this case on the vertical mounting planes), with some separation of the PCB backside to let the heat flow thorugh the ground pads (not unlike the S5).

I really should get a 3D model because i don't seem to make sense with words today (had my birthday celebration yesterday and i'm a little verbally impaired atm )