We have Single Sign Implemented here and it is going well except when new Chatter Users are invited by another User to join Chatter.  I am doing some preliminary research to determine if we can overwrite the standard Chatter 'Invite Co-Worker's Functionality.  Some high level requirements:

1) Replace current 'Invite Co-Workers' button with a custom button.

2) Current User clicks on new 'Invite Co-Worker' button and fills out the 'Invite People to Chatter' form.

3) The invited user should receive a modified email template that includes our custom domain (not login.salesforce.com).

4) When the User clicks on the URL in the email template, they should be directred to a custom landing page that requires them to enter First Name, Last Name, Title, at least 1 Phone Number, and select the correct timezone.  (Many Users don't bother verifying the correct time zone, so we would rather have no default time zone and make it a required field.)

5) Somehow, behind the scenes the invited User's user record's Federation ID is updated with the correct ID (it may not be the email address original entered by the user sending the invite.

Has anyone done anything like this before?  Any suggestions?