I think the problem here is probably that the genericdao framework is not able handle the @ElementCollection case. I didn't think of this case before. Thanks for bringing it up. Perhaps I can add it to the framework in a future release.
In the mean time, write your method to use native Hibernate querying. You can get the Hibernate session directly in your DAO using getSession(). When you get it working, please update this thread with the HQL query that were able to get working. I can use that to figure out a way to build the capability into the framework.
Thanks.
-- David