Hibernate Search today relies on Hibernate Core. If someone is interested in constributing, abstraction might be a doable task and certainly a good exercise for JPA 2.0
Integrating Compass and Seam is different. Hibernate Search extends both the Sesison and EntityManager API to provide a unified view to the user. Compass querying model is different, it has it's own Session-like object.
So integrating HSearch was easy, since it just extends existing frameworks integrated with Seam esp JPA. For Compass, something different would be needed.