Thanks for the response and the resource--I think I'm using that same methodology to put the processor into powerdown mode. The 328 should only be drawing minimal current with everything powered down and ADC off--but something (or many things) elsewhere on the board are contributing to the 6mA draw while the 328 is powered down.
I'm curious about what hardware components could be responsible, since the voltage regulators, LEDs, and USB interface (16U2) have been removed. Could the op-amp or some other component on the board be generating this quiescent draw?