I don't have the game, but it sounds like GPS power drain is the big problem. Is there not some way to improve that? Maybe only turn on GPS intermittently and use accelerometers, wireless signals or the camera to track your position from the last known point and any landmarks?
Of course, if you don't already have one a Bluetooth GPS receiver appears to cost at least 5x as much as a low-end external battery and 2-3x as much as a pretty substantial one.