If you could get them to bark on cue (perhaps with addressable collars) then you could triangulate the position of the sound. Of course if they all start barking in a pack, then you're back to zip.
Actually, that's not a bad idea; if you replaced barking with something else.
Perhaps a collar or harness with one or more ultrasonic transducers, that is addressable, and can output a "ping" when addressed remotely. In the "outdoor fenced area", you place a variety of "listening" stations, each one anchored to a predetermined location (via GPS); they might look something like a light pole, with the sensor (ultrasonic receiver) above (about 10-12 feet above the ground), perhaps with a reflector or something. These poles would be spaced apart in a hexagonal grid, perhaps 50-75 feet.
Your transducers would have to be powerful for the transmitter and sensitive for the receiver, but the idea would be that you would "ping" a particular collar, and the various stations near the dog/collar would each receive a pickup of varying strength; when they did, an interrupt would be triggered, and they would relay the information (strength level and reporting station number) to a central base station (likely PC-based for data storage, and mapping). Triangulation (with knowing the locations of the receiving stations) using the readings would be able to tell you the rest...
It certainly wouldn't be cheap to set up (differential GPS would probably be cheaper).