Someone mentioned, earlier, the golf course incident. The same or similar was at Cranfield, PFA Rally a few years back. Kitfox with a dead engine. That incident made me think of an emergency horn too. An air powered one, as is used as a foghorn on light sailing craft would do nicely. Trouble is, it probably wouldn`t work. The `targets` would look all around them, under trees and bushes and probably down the hole but they would never dream of looking up.
Glider pilots land in fields etc all the time but these landings are pre-planned once the glider is down to about 1000`. That is several minutes in a glider but only a few tens of seconds in most unpowered powered aircraft.
Mike W