In the same way if the person in charge of the boat happens to sail at the sharp end of said vessel, the rules allow them to change the tiller waggling crew as long as they obtain prior written approval of race committee.
In a multi-event series the boat can sail any event with anybody on board – as they are competing primarily in the event. If the designated person in charge was not on board, it It would then be up to the organising authority of the series (proabably UKWA in this case) to decide if the boats score for that event shouldbe considered as counting for the series. It would be best to write any provision into the NOR of the series
I would suggest that any NOR for a multi-series event should stipulate that the designated person in charge should compete in an event for the event to count towards the series. Any change to the designated person in charge should be approved by the SERIES organising authority.