This is probably obvious, but I thought I would throw it out there. I don't know anything about speeding up single copies, but since all your drives are in full systems you could do it in 4 iterations (16 hours, instead of 48 hours; assumes you have enough Firewire cables and power connections (and air conditioning
)):
Machines are labeled "A" through "L"
First Iteration: A -> B
Second Iteration: A -> C, B -> D
Third Iteration: A -> E. B -> F, C -> G, D -> H
Fourth Iteration: A -> I, B -> J, C -> K, D -> L
After each iteration, just start the freshly duplicated system(s) and use them to copy their own drives to another system in the next iteration. Since you have eight systems after the third iteration, you could actually do eight more systems in the fourth iteration, but you said you only had twelve in total.