If you build your own standalone programmer you could setup all of their ICSP pins in a parallel bus except the reset lines. Each reset line will need to remain separate, then using some kind of multiplexer selector, you would select one reset at a time to program that unit. You could use an Arduino running a modified version of Nick Gammon's hex uploader (Gammon Forum : Electronics : Microprocessors : Atmega chip stand-alone programmer to upload .hex files) that could run through and automatically select each unit one at a time (using the selector) to program very quickly.