I can see some business problems with this. You got bad reviews for the system being too quiet. How are people going to respond to a system that keeps cutting out?
Also if it was me I would first of all have a member of MY staff there to monitor the venue, I dont like the idea of some drunk with a beer in his hand messing with me equipment! The other problem I can see is the person who moaned in the first place is still going to moan if the noise keeps oscillating, as in one min its loud then it goes quiet then it goes loud etc etc. From a business point of view why not have the suggested master switch in something like a key safe thats locked, meet your client on the day of the event explain the noise level issue (you should have done this before they book as well) and together set the master switch to an agreed level (as long as its within the law).
That way the customer cant moan the sound is too low and the neighbor cant moan if your within the limits. Putting something on that turns off the way you suggest is likely to get you worse reviews, most people at the function wont have a clue there is a noise limit system in place, they will assume the sound system is just broken and keeps dropping out. By all means build something but this to me is going to harm your business not help it, and I still dont get why you dont have staff there looking after your interest during an event.
Just my thinking thats all