while attempting to construct a spawner that would spawn stationary ghast fireballs, I spawned a ghast, switched to survival, and paused the game right after the ghast shot a fireball. I saved the fireball as a schematic. I opened the schematic with NBTedit, and changed the velocity data to:
I relogged, but the ghast fireball still moved in a single direction. Upon creating a spawner for the fireball, it spawned fireballs that moved in the same direction each time that they spawned.