I'm not sure if that would work, because once TurboCNC closes, it no longer controls the pins. Why couldn't the drives end up back on again? This is not the same as TurboCNC's enable lines. Some drives have enable lines that need to be on for the drive to work, even if it's powered up. That's what the enable is there for.
I believe you can do this in Mach2 with this breakout board. http://www.campbelldesigns.com/breakout_board_plus.htm