I would really love to make a tennis court with this kind of function. Unfortunately, I'm unable to test this out at the moment, but would like to know for future reference. Has this ever been done? Is this an original idea? Am I a lunatic? I only know the answer to one of these questions.
I agree that the answer is probably yes, based on the Wiki "The ghast's fireball attack has a straight trajectory unaffected by gravity and a theoretically infinite range, although it disappears after 1 minute" and "] Fireballs do not track the player once fired, are slow, and can be deflected if the player hits them with a projectile, fishing lure or melee attack" (though, of course, I already knew the latter) and not seeing any reason to think that an already deflected fireball couldn't be deflected again.
However, my own usage, reading the subtitles of a few of the YouTube videos and watching the first one all point towards "Ghast tennis" normally referring to hitting the fireballs back at the ghast.