I still wonder why you need to follow the selection when you have no UI - especially as you also seem to target InDesign Server.
Anyway, InDesign itself puts plenty observers on kSessionBoss, so you could follow that example. Several others are on kAppBoss. Other observers are directly aggregated on a startup service boss that you'll probably need to invoke AutoAttach - plugin load time would be too early because the object model won't be ready at that time. It all depends on whether the observed subjects are already available.