From the xdoclet-user mailing list:
I just uploaded a snapshot of xjavadoc that (among other fixes)
provides support for Java 5, as of XJD-41. Many thanks go to
Anton Adamansky for his work on this issue.
You should be able to just replace the xjavadoc-1.1.jar from a Xdoclet binary release with this snapshot and use jdk1.5 features with Xdoclet.
Let us know if you find any issues.