While working on, I discovered (with help from @chx and @tim.plunkett) that it's impossible (at least without doing a bunch of black magic) for an annotation class to extend another annotation class if they reside in different namespaces. This is a major gotcha for anyone who wants to define a new annotation by extending an existing one.
AnnotatedClassDiscovery should accept an array of additional namespaces in which to search for annotation classes.
^^ Write that, in patch form. Commit heartily, then sit back and have a drink.
Additional argument to AnnotatedClassDiscovery::__construct() and greater flexibility in defining annotations.