Where can I find regulations about when live ranking tables may be shown?
EBU intention for its own events is only to display results when player have played the same boards - end of swiss matches, end of the rounds in barometer events, but only at the end of a session for ordinary pairs. We also display results for teams events when head-to-head matches are completed even if the boards are still live.
This isn't written (in the White Book) because it is not a regulation the tournament committee/panel thought was a policy clubs or counties would necessary wish to adopt.
Many clubs do have policies - as soon as possible / from half-way / for the last two rounds / only at the end.
For anything between clubs and the EBU, they may not have a policy and it is best to ask the tournament organiser.