we made some of these a few years ago for a company in london its quite easy really from the electrics side of this you will need a relay for the radio side of this you will need a reciever that recieves air band transmitions and either a tone burst generator or a circuit that would receive a set number of mike press,es to activate the relay
the grey area is if you transmit on air band even if you dont speak you may stop others using their own legitimate frequency
you could use a separate legal frequency walky talky type radios but the range would be no more than 2 miles at best
either way it definatly possible for minimal cost
steve