Did WW2 airplanes ever jettison their empty external fuel tanks to act as weapons?
Yes, and not even empty tanks. It didn’t always work, but it’s happened:
     This is how American pilots used drop tanks as bombs during WWII.
Fun fact, cellulose drop tanks were developed to save metal during the war! The article references P-47s, but I would not be surprised if P-51s did the same.