I had this exact play happen in a LL majors game. I am a BB referee that also does some LL games. What I did was to judge the line drive a foul, It would have landed foul, no one was in position to make a play on the ball, 3B was playing in front of the base anyway. I killed the ball, had a foul and made sure the runners did not advance.
I had thought that a runner could not be called for interference when legally on base? am I missing something? Thanks for the help!