I have experienced this state a few times on psychedelics.  I have even referred to it as the Godhead. 

Buddha, however, takes it one step further.  The logic would go something like:   For awareness to exist there must be an object (object in this sense can be anything.  A thought, an emotion, a tree, river, solar system..etc) to be aware of.  So eternal awareness would still imply permanent duality.   It is said the first Jhana in Buddhism is equivalent to giving up the doer.  The next 7 Jhanas are progressive levels of giving up the witness. 

My thoughts are that in the end there can't be a witness if there is nothing to witness...or if the witness is only witnessing itself...then it doesn't really even make sense to refer to it as a thing or a nothing.  It is the only.  There isn't an other or an it.  There is only.  To say more about nirvana is to essentially lie.  The best anyone can really do is say what it's not.  It's certainly not an eternal witness staring at itself... that would eventually get boring in my opinion.  Besides the witness arises in conjunction with the doing.  If there is no doing there is no witnessing.  Which implies to me that both the doer and witness are constructs of dualistic thinking.  

Both doing and witnessing are one and the same.  One depends upon the other.