Which?
So to paraphrase the first two your argument using equivalent terminology:
1. Mental states are identical to brain states
2. It is possible to have complete knowledge of brain states without having a complete knowledge of mental states.
Yes?
No. If brain and mental states are identical, then complete knowledge of one is complete knowledge of the other. Otherwise, they're not identical. If John is genetically identical to Bill, then knowledge of John's DNA is knowledge of Bill's DNA. It would be impossible to have complete knowledge of John's DNA and be missing some knowledge about Bill's DNA.
And again, if we don't expect a photograph of water to be wet, why would we expect an MRI of a brain experiencing the taste of salt to be salty?
In other words, how do you get from 1 to 2?
Good point. Two points of my own:
1. Can you predict you will get a compound with the property of wetness just from knowledge of chemical bonds, electrons, Oxygen, Hydrogen, etc.?
2. "Wet" is a property of water, it's not identical to water, so I don't think this is analagous. The claim isn't that a mental state is a property of a brain state- the claim is that a menal state is identical to a brain state. Is it possible to learn about a brain state without actually having the brain state in your own brain? Sure. In the same way you don't need to spill water on yourself to learn about water.
How much of a particular brain state can you know without experiecing the brain state? You can know exactly which neurons are firing, where they're located, how much of chemicals X,Y,Z are present, electrical activity, etc. The better question is, what
won't you be able to learn about a brain state without experiencing it? What bit of information will be forever hidden from you because your own brain state is different from the one you're studying?