Ability to choose contact's email address if more than 1 in system for Email Marketing

For example - when customers/trialists sign up to a Webinar, I send out a follow up email after the session with the recording. However, the email address they use to sign up for the Webinar isn't always the same as the one that is in the CRM. Not sure exactly how this could work, but maybe when you try to add that contact to a Campaign Activity, it could flag up a message saying 'do you want to use this contact's primary email address, or X email or X email'.