F5 Lite Maintenance Page iRule works on Virtual Servers except when the Virtual Server is Listening on 443 (HTTPS)

If you don't need images, then it's probably easier to ignore the URI inspection code and just look at active_members:

when CLIENT_ACCEPTED {
    set default_pool [LB::server pool]
}
when HTTP_REQUEST {
    if { [active_members $default_pool] < 1 } {
        HTTP::respond 200 content "...your html code here..."
    }
}