If you're using Seam Faces you can @Inject an EntityManager. People are typically doing a Seam Managed Persistence Context for this. Another way would be to @Inject another component which has the @PersistenceContext in it.
Yea, I ended up injecting another component that uses @PersistenceContext.
As far as I get it, one can't use @PersistenceContext in a validator because it isn't a bean managed by the container.
Anyhow, Thanks Jason for your reply.
Greetings from Germany