$zend_authNS->setExpirationHops(1);
Would be the normal way to expire a session on each page request.
You seem to be using Zend_Auth, so I wonder if this can be accomplished using the storage component of Zend_Auth, maybe:
Zend_Auth::getInstance()->getStorage()->getNamespace()->setExpirationHops(1);
Haven't tried it but it might work.