I have thought of this for a while and I think that it would be possible for Zwift to do this since all the data is allready in place.
Would it be possible for the event crew to add a new little feature called “Zwiftpower race” or something like that. The feature Iám after is that any organiser that organise a race should be able to set this option to ON in a race they organise. What Zwift then would do is to check weather the rider has opted in to Zwiftpower or not. So either when a rider signes up to a race in the companion app or when they enter the race the would get a sign saying that they need to opt-in to Zwiftpower to be able to race in that race.
So Zwift allready has the information if a rider has opt-in to ZP or not so adding on this feature would be a good way of getting more riders to opt-in for ZP races, and together with the new “Anti sand bagging” feature this would give us better control over the races.