I have designed and built 8 Channel boards to do just this (with indicators lights and a lot of other bells and whistles). I am not ready to offer these up for DIY consumption just yet...
In any case I suggest using DPDT toggles that are in the ON-OFF-ON configuration.
POLE 1 will be used to switch the HOT
POLE 2 will be used to switch the NEUTRAL
WHy? better safe than sorry and we don't want any other wiring mistakes to energize the neutral or accidently tie two branch circuits together (AC jur plugged into one circuit, your manual power into the other).
Now for the HOW:
Simply wire the DEVICE to the COMMON TERMINALS of the switch. Wire the OUTPUT of the ACJR to one set of terminal and mains power to the OTHER set of terminals.
FLIP the switch UP and the AC power comes from the ACJRs output, the device only runs when the ACjr tells it to.
FLIP the switch to the center and the device is OFF no matter what the ACJR tells it to do.
FLIP the switch DOWN and the device is ON no matter what the ACJR tells it to do.
I would get 15A toggles to be safe (20A if it is a 20A breaker).
You can also look for HOA (Hand Off Automatic) switches on ebay. You want stationary switches, not spring return. These are the big switches you find on the front of big equipment and elextric switch gear. Allen Bradley, GE, Cuttler Hammer etc. They will do the same thing, but look industrial and be a lot more durable.
Bean