I like this suggestion from @Stuff_the_Second in the comments, opening this thread for further discussion and ideas (and so I don't forget):

"I suggest making an event that triggers the provocation but only firing after a certain amount of time has passed since the formation of the league, somewhere around a MTTH of about 50 years, with the time being reduced based on how many Protestant, Anglican, Reformed or Hussite nations have already join the Protestant League."