Since this is an unprecedented event, I don’t believe that there’s anything specifically in the conference bylaws that prevents teams from embarking on a temporary “independent” schedule when the conference opts out, but there may be some tangential prohibitions. When I was in the Navy, there was a charge of “Conduct Unbecoming” that seems to be a “catchall” if they wanted nail you with something, so there might have been some kind of language that would either prevent it, or enable the rest of the league to sanction (or expel) members who don’t toe the line.
The BigTen found a way to bully Nebraska into getting on board, and since the Pac-12 vote was “unanimous”, there shouldn’t be any league mavericks looking to go rogue in 2020.