I want to distract my doubts for performance issues on big meta-models with the proposed filter: on modern cpu's it processes way more than 100 classes per millisecond, so it's really not an issue. Re-putting the filter as proposed in the description would really fix the think for us and all people which deal with interfaces instead of direct using the entity type as query return type.
The concept behind filter is easy and sound IMO:

• if the passed class is a mapped entity class, call addEntity
• if the passed class is not a mapped entity class, don't call addEntity because it does the wrong thing and will lead to an exception.
  Please let me know your intentions about how to proceed with this issue.
  If required I can change this issue from a bug, into a requirement.
  regards
  G.D.