ZwiftPower should introduce a "verified" profile, somewhat similar to Twitter's blue checkmark, or how Paypal requires your credit card to be verified in order to fully unlock all usable features of that card.
In its essence, you probably have to submit some kind of document (perhaps a medical document or something?) or video, that their staff will look and say, "okay this dude's height and weight are authentic" and mark that account as verified. Race organizers can then place requirements around the verified status the same way they filter off zPower for certain levels of races.
Weight verification probably needs some amount of refresh; maybe its verified status has an expiration date, that the user must regularly update. Or at the very least is able to show as "expired", and race organizers have additional control over that.