Looks like inheritance was added to implicit collections about a year ago (http://svn.xstream.codehaus.org/?rev=517&view=rev), so I am creating a new jira for the inheritance side of XSTR-295 (http://jira.codehaus.org/browse/XSTR-295) and including a better patch.
I did not apply the patch, since I wanted to keep the existing (simplier) key, but it works anyway. Your unit tests work (also the one with the overwritten alias) and I ensured that the functionality is also availabe for the SerializableConverter and the JavaBeanConverter. Give the head revision a try, if you wanna test it yourself.
Thanks for your patch,
Closing issues before next release.