Unfortunately i don't know of any Asic miner with software shutdown, all they can do is restart, and if you go the
Braiins OS firmware route, you could in theory reduce its consumption significantly but not completely turn it off. If it is of any help, think that each miner could change from pulling 1400w into 360w with that trick, depending on how many you have, it might be easier to do it this way.
All you need is to do is run a script in the miner itself to poll the power condition, and act upon it. In your case i would have two cgminer.conf files ready, one for normal condition, and another for low power condition, and switch accordingly. The script would just run off cron periodically as often as you want (just don't run it every second, please). Software changing the speed is rather simple and painless, it only restarts cgminer.
The only other alternative is as you said, turn on and off the PDUs. If you go that way, try to use different delays for each PDU, avoid having all of them go on and off at the same time...