Is it possible to hook into the post-shutdown-process of win7 (like updates do)?

No, you can not do anything except cold shutdown your PC, which would require you to either unplug or hit the killswitch *Holding Power Button for 3 seconds* There is no ACPI after Shutdown, once you shut it down, it's going to shutdown. The security updates are configured before shutdown when you download them and are applied then.