I have talked to Bill a bit about this, and decided to go with his advice: suspend the entire motherboard from the ceiling via twine. My implementation is sort of a "hanging plant" style setup, and it is a good bit lower than the test equipment (actually it's pretty near the floor). This way, the wb inlet and outlet tubing can be pretty close to vertical and minimize the strain that the tubing puts on the block both as you clamp it down and later under use. If you have a Swiftech block, then use those springs for all blocks, as BillA has a load-defection curve here:
http://www.overclockers.com/articles608/
He told me the number of turns for proper force; I want to say 7 full turns past first contact.
If you get the same amount of even pressure and no strain from tubing, you are left with variability of the TIM joint. That's where no cleverness in setup design can help.