If they can manage to keep the bottom clean they could definitely use a back plate to help cool the board.
Most QFNs have epoxy from the top of the die to the top of the package. It might not be a great thermal interface, but given the very low power per chip it will still be able to help.
Obviously the biggest constraint is time-to-market or time-to-mine.
Wouldn't it be practical to keep all PCB layers clean under the ground paddles; use a hole punch to cut out the PCB epoxy under the chips and mount the real heatsink on the bottoms with copper washers/thermal spreaders? The tops would then just need a mechanical clamp.
I'm just brainstorming. I made a suggestion about 5 months ago about using power IC packages in this thread:
https://bitcointalk.org/index.php?topic=91173.msg1062969#msg1062969 . friedcat responded then immediately deleted his response because it probably disclosed something NDA-embargoed.