Is there a way to automatically set the value of a User Profile property right when the user first visits a SharePoint site -regardless of what page they load first? I'm kind of looking for the equivalent of a "UserAuthenticated Event Receiver".
One way you could do this is by writing your custom code in a user control and adding it to the master page of your site. Have it run code in OnLoad and use
DateTime flags to detect whether or not the property has been updated per your requirements. This will guarantee every page using the master will run the code, not just the landing page of the site collection.