You do present an interesting question , in that what your looking for is SO simple that I don't know if it exists in the model aircraft/model car RC world . Your looking for a "single channel" device with no proportional control , a simple on / off . With RC equipment each separate thing you want to control requires a channel , for a model plane it would be one for the elevator , one for the rudder , one for the ailerons , and so on . The most simplistic , toy like RC devices we have are 2 or 3 channel with of course the controls designed to be proportional movement of a rudder or elevator (or steering and forward/reverse in the case of RC cars) based on the movement of the TX's sticks . We do have available a relay based device that can be plugged into an unused channel (it's VERY common for our radios to have 7 or more channels) that can provide a switched set of contacts for landing lights or other powered accessories but you would still need a simple single channel transmitter and receiver to activate it . I believe by the time you price doing something so simple with RC parts of the sort you'd find in a dedicated RC venue such as this you'll find it cost prohibitive , and I wonder if a possible modification of one of those remote light switches of the sort the hardware stores sell might not be a cheaper alternative to using RC parts ? If not that , a possible modification of a "toy grade" RC would be your best bet , of course either of these options would require the services of someone well versed in the tinkerings of RC electronics .
As a last thought , if no one pops in here with a mission perfect device that I'm unaware of , you may hit up the Robot building forums , those guys with their electronic/RC knowledge are likely to know far more than I about how to adapt something to work for your project .
Good Luck with it ...