If Ironman wants to limit the number of BoP pros (in the absence of federation rule changes), the solution is simple: create a filter.
And since they have the pro series as well as non-pro series races, it would seem to me that the easiest course of action is to use those other races as a way to filter for the pro series. Give those races half as many points as the pro series (but probably with a slower point decay) and then use the combined points from the past 12 months to determine who gets into Oceanside and the other races with too many pros (however many you want).
Exceptions can be made for anyone who’s podiumed at a WC in the last 5 years or maybe just go with your usual wildcard rules. (So Wilde or Waugh can get in)
If you really want to get fancy, you could throw in a penalty (not to the pro series but to the filter) that docks you for DNS within so many days