Experiencing the same lack of field.
i'm only able to set the label of filter operator if I expose it.

@pinkonomy, did you find a solution for your problem? I'm looking for something simular...

Hi Have used the below php code to find the distance between two points:-

function distance($lat1, $lon1, $lat2, $lon2, $unit) {
$theta = $lon1 - $lon2;
$dist = sin(deg2rad($lat1)) * sin(deg2rad($lat2)) + cos(deg2rad($lat1)) * cos(deg2rad($lat2)) * cos(deg2rad($theta));
$dist = acos($dist);
$dist = rad2deg($dist);
$miles = $dist * 60 * 1.1515;
$unit = strtoupper($unit);
if ($unit == "K") {
return ($miles * 1.609344);
} else if ($unit == "N") {
return ($miles * 0.8684);
} else {
return $miles;
}
}

We just needs to pass the lat and long of both the points. Gives a pretty accurate results.