Random thought/question: couldn't Roshi or Piccolo use mafuba to ring out any opponent since the tournament's rules effectively make the outside of the ring a container? Not that I expect them to but it would be pretty trollish if they just ring out Jiren that way.
For Roshi it would probably drain a huge amount of his stamina. He mentioned it in the one episode, so I assume he'll use it at some point. But it's something he'd use when he's nearing his limit.
Same for Piccolo.
They don't have to seal them, they can just throw them out.