Well I think in theory this should be possible without actually changing something in monowall.
An own created portal Page with a javascript function setting a cookie with the session ID and a small php script uploaded per filemanager should do the job.
php script should work like this:
include monowall functions
check if cookie is set
not set, redirect to login page
set, call monowall logout function and redirect to a logout page
Haven't tried yet, but came to me when I read the topic. Will test in the next few days...