DISCLAIMER: Potterworld or Droobledore LLC is not in any way affiliated with Mojang AB, J.K. Rowling, Warner Brothers or any company, copyright or trademark.
This may have already been suggested, but when you through a practice quabbleball on a broom it dismounts you, which removes a lot of practice potential and basically the entire point of the ball. So I would suggest that it doesn't dismount you.
As Matty indicated, this has already been suggested here. We unfortunately have to decline this suggestion as it isn't possible currently.
Nevertheless, thank you for suggesting this again and have a great day!