WCF Custom Certificate Validator: How to call built-in chain trust validation on top of your custom validation?

I have a custom validator that does validation b开发者_开发知识库ased on the thumbprint of the client cert. How can I still call the in-built chain trust validator or perform chain trust validation?

Thanks.

Dominick Baier describes an approach that works here: http://leastprivilege.com/2007/08/25/certificate-based-authentication-and-wcf-message-security/