Derek,

You say: "My 'principled reason' for claiming that 'consciousness can not simply be brain states' is simply that it makes no sense to say this."

From what I gather, your view is that it makes no sense to say anything at all about consciousness, except that we don't know anything about it.  So it should make just as little sense to you to say "consciousness is not brain states" as to say "consciousness is brain states."

As I think I told you already, I share your concern about the how the term "consciousness" is sometimes presumed to have a clear meaning in philosophical discussions.  The term "consciousness" has many uses in our language, and the fact that we understand ourselves as conscious beings is an integral part of who we are; but this does not mean that "being conscious" means anything in particular, or that the term "consciousness" must refer to a particular entity or process. 

We should not betray how the language is used in normal (i.e., non-philosophical) life--not only the word "consciousness," but a family of related terms, such as "feelings" and "experiences."  To understand consciousness, we have to understand how people use the language of consciousness--because an explanation of anything else would not be an explanation of consciousness.

For example, if our language of consciousness is clear on anything, it is clear here:  that we talk of a person being conscious when they are awake, and not when they are asleep.  Neuroscientists have made headway in understanding how sleep-wakefulness cycles are regulated.  The science of consciousness is already advancing.

There are countless other demonstrable relationships between neurological processes and consciousness.  At the simplest extreme, when you close your eyes, you no longer experience vision.  The more we understand the brain, the more we understand the behavior we associate with consciousness; that is, the behavior which motivates our use of the language of consciousness.

I know you are skeptical of a behavioral approach here, but I hope you will consider subjecting your prejudice to further scrutiny.  To say that consciousness cannot be understood in terms of behavior is to say that the language of consciousness cannot be learned; which is to say that all talk of consciousness is meaningless.  But if that were the case, the problem would not be that consciousness is outside of our scientific grasp; rather, it would be that we were all deluded in thinking that there is anything to grasp for.  The everyday language of consciousness would be a widespread error, and nothing more.  (This is not a view I encourage, but it follows logically from an anti-behavioral stance.)

As I understand it, there are no a priori limitations on scientific discovery.  We cannot use philosophy to describe something which exists but which scientists cannot understand.  So there is no basis for postulating "something else" here, if by that it is meant something beyond the realm of scientific discoverability.  Perhaps consciousness is not all about neurological function; but that should be a scientific hypothesis, not a philosophical presupposition.