I've added a dependency on an Ant Task in project/build/plugins/plugin[artifactId='maven-antrun-plugin']/dependencies/ and run that anttask using the antrun plugin.
When run from the project dir itself it runs fine.
When running from the root of the project tree (reactor build, project one level below root),
antrun bails out because the taskdef can't be found (not on classpath).
It looks like the dependency isn't resolved, or not added to the plugins' classrealm.