I think you are misunderstanding what modrewrite is capable of.
modrewrite can be used to convert query string variables into friendly urls, for example:
but the query string variable still needs to be in the URL for the php server to be able to use.
if you created a rule to strip the UserID variable, it would just redirect to the UpdateBanner.php page with that variable empty.
With the user registration solution pack, when a user logs in, a session variable named "UserID" is created that contains the Id of the logged in user. Why not use the session variable on the UpdateBanner.php page:
instead of the passing it through the query string?