The thing is that chaotic systems, though fully deterministic, are highly uncorrelated in time. While the state in a chaotic system at time n might be entirely dependent on all the times steps (n-1), (n-2) etc, in a purely mathematical sense, functionally there is almost no connection between the state at a given time point and the time points much earlier than a few moments. Take the weatehr a for example. The weather today might have high correlation to the weather tomorrow but it will have almost zero correlation to the weather in two weeks from now.
Now if the brain is a chaotic system, then proving it is deterministic really alters nothing in the context of free will.
If you want to argue about a lack of free will, I think the effect of genes and the social environment are much stronger argument paths.