I came to a similar conclusion and ended up with this:
if ( is_page( 'edit-ad' ) ) {
if ( isset( $_GET['id'] ) ) {
$ad_id = $_GET['id'];
$location_values = get_field( 'sd_city_selector', $ad_id );
$field['instructions'] = '<strong>Your location is: ' . $location_values['cityNameAscii'] . ' (' . $location_values['countryCode'] . ')</strong>';
}
}
I wanted to populate a message field, but for space I decided to use the instructions field instead.