ArtifactRepositoryLayout isn't extensible

I'm writing a plugin that (among other tasks) provides a new ArtifactRepositoryLayout suitable for Eclipse repositories. In my META-INF/plexus/components.xml, I have:

<component>
<role>org.apache.maven.artifact.repository.layout.ArtifactRepositoryLayout</role>
<role-hint>eclipse</role-hint>
<implementation>org.apache.maven.artifact.repository.layout.EclipseArtifactRepositoryLayout</implementation>
</component>

This plugin is listed in the POM with <extensions>true</extensions>. I would expect that a <repository/> with <layout>eclipse</eclipse> would discover and load this layout provider. Instead, it simply results in an exception. Note that this same components.xml file also defines some lifecycle mappings, which work perfectly.

[INFO] Trace
org.apache.maven.reactor.MavenExecutionException: Cannot find layout implementation corresponding to: 'eclipse' for remote repository with id: 'Eclipse Update Site'.
at org.apache.maven.DefaultMaven.getProjects(DefaultMaven.java:365)
...
Caused by: org.codehaus.plexus.component.repository.exception.ComponentLookupException: Component descriptor cannot be found in the component repository: org.apache.maven.artifact.repository.layout.ArtifactRepositoryLayouteclipse.
at org.codehaus.plexus.DefaultPlexusContainer.lookup(DefaultPlexusContainer.java:323)