I suppose zombiephiles could simply posit some sort of non-physical observable, but then they would be just piling on further unsubstantiated claims for the sake of preserving the zombie-possibility doctrine. They could adopt epiphenomenalism, but that means accepting that the apparent causal effectiveness of qualia is illusory. They might adopt a solipsistic stance and say that their conscious experiences are the only things they can be sure about, but that comes with a hefty dose of motte-and-baileyism: you doubt the external world, but you're sure about zombies?
On Chalmers' "additional natural law connecting consciousness to information rich processes": Firstly, proposing something is cheap; turning it into a verifiable hypothesis and then following through is hard, and Chalmers et. al. seem to have no interest or intention of doing that, preferring instead to endlessly repeat their anti-physicalism arguments. Secondly, there is an attempt to redefine what's meant by 'physical'. The physical was originally defined by what's tangible to our senses, and as additional phenomena having a causal connection to the physical world were discovered, they were naturally subsumed into an expanded concept of it (gravity, for example, despite Newton's unease over doing so.) Chalmers is free to propose causes that lie outside of our current concept of the physical (and he's even free to look for them!) but to insist, without any justification, that they would be non-physical, is at least tendentious and probably question-begging. To settle the question, we need more facts, not more speculation.
I'm not sure what to make of your last sentence: the p-zombie and Mary Room arguments are indeed intended to be critiques of physicalism, but as you have rejected the possibility of zombies, I assume you do not take them both to be successful arguments. One thing about Mary's Room is that you can make essentially the same argument, but with Mary's prior knowledge including Chalmers' additional natural laws.
If so, then consciousness is physical, if not, then it is non-physical.
> In particular, if one accepts the possibility of p-zombies, one must logically accept that no explanation of consciousness, regardless of how thorough and well-supported it seems to be, has any basis in any facts about consciousness.
Although I disagree somewhat here, because logical isn't the same thing as physical. All you can say is there is no physical explanation of consciousness. But you could logically come up with one. For example, Chalmers has proposed an additional natural law connecting consciousness to information rich processes. Natural doesn't necessitate physical, if the universe contains more than physical stuff.
Or if physicalism is the wrong ontology of nature. I take the p-zombie and Mary Room arguments to be critiques of physicalism.