Does ADS-B *need* to be transmitted from a model flying site location, or could it be broadcast from a few miles away, perhaps from a local airport? The signal goes quite a long way and contains the GPS co-ordinates.
If a local airport could transmit ADS-B on behalf of model aircraft then perhaps us aeromodellers could simply log on to a website and state that we will be flying at location X for the next hour or two and then the airport ADS-B transmitter could broadcast that for us.