well let's use "brain activity" instead of brain state
If you mean neuron impulses, it seems to me that you don't need QM for that. There are other subspecialties in human biology that have studied that subject explicitly and more closely.
someone who holds the idea that mind / cognition / consciousness is wholly reducible to physical processes.
Not quite true. The idea behind much of science is that you must be able to produce predictions that can be tested. QM does this very well and has passed that test every time.
Quantum mechanics was specifically developed to model the behavior of subatomic particles, it has absolutely nothing to do with "brain states", emotions, or the like. If you want to call that "reductive", so be it. It works very well for its purpose and not for anything else.
If you can't make specific predictions that cannot be compared with data, all you are doing is some version of this: Image: https://dl.dropbox.com/u/44760674/TD/Gifs/mind-blown1.gif