We received a neat hack from PCA Northeast Region for password protecting a portion of a registration form. While we don't support this natively, combining an event question with a validation rule that checks for a particular answer lets the registration form stop the user if they don't provide the right password.
This is a clever way to provide an option for pre-approved or in-the-know members only. It could be used to protect a single option or the entire registration form. Very cool! We've added a setup guide in the documentation here.