Possible to join different entity type on using EntityFieldQuery? I am wondering whether I can use EntityFieldQuery to fetch all entities belonging to a specific content type. This entity type has a reference to another entity type. Is it possible to check against the refereed entities fields in the same query?
No. 

An EntityFieldQuery allows you to run queries based on field and property conditions, but it has no way to query the fields or properties of a referenced entity.

Such a thing would involve some pretty full on logic, and it probably wouldn't even be possible to provide a programmatic interface to it that made sense.