How to enforce a persistent URI

Instead of using URI to determine which portal site to go to, why don't you use a different host names?

"customer".portal.com where "customer" is the name of the customers company, you can still have a single virtual host and have both host names point to the same IP address. Set the cookie to the same value as the host name when they originally connected and then just compare the value of that cookie to the current host name in the HTTP request.