LTM Maintenance Page - HTTPS

If you aren't terminating the SSL on the LTM, then you aren't going to be able to provide a direct maintenance page response. You can however direct client traffic to another pool in the event the one assigned to your virtual server has no available members. Ideally this maintenance_pool would contain one or more members that would serve up your maintenance page. Something like this:

when CLIENT_ACCEPTED {
    if { [active_members [LB::server pool]] < 1} {
        pool maintenance_pool
    }
}

Terminating SSL will give you a wealth of additional options, if that's a possibility for you.