I have started a Realm implementation. A first version (proof of concepts) already works quite well. I have committed the sources here as a Maven2 project:
It builds a jar file which contains the Realm class. In order to use it, one has to copy it to the Tomcat server lib directory, along with the required ApacheDS and dependency jars.
Activation in Tomcat server.xml is simply
<Realm className="org.apache.directory.tomcatrealm.EmbeddedApacheDsRealm" />
You can use Studio to edit your users and groups after Tomcat startup; the embedded ApacheDS opens an LDAP port at 10389.
I will continue testing and will provide some documentation on the wiki later on. Some features like configuration (LDAP port, filter expressions for searches, ...) are still missing, but I think it looks quite promising.
Feel free to check it out and provide feedback. Further discussions about whether we provide this as official artifact will be on the dev-list, which you hopefully scan as well.