I agree that on approach and departure ATC could get signal broadcast by drone and notify aircraft, but any drone flying in class B,C,D airspace has to get LAANC approval before hand.
There are ADS-B Out for drones ( certification pending) but this will work only, I believe, if above 500’. I don’t think they will ever require it for drones limited to 400’ as this will probably clutter the ADS-B In screens on any aircraft.
ping200X TSO Certified Mode S ADS-B Transponder for UAS ping200X is the premier FAA TSO Certified Mode S ADS-B OUT transponder for UAS. Enabling airspace access, regulator and Air Navigation Service Provider (ANSP) acceptance worldwide, ping200X enables your aircraft to be detected by...
uavionix.com