Hi IMark,
The fact that theses researchers had to convert NAC into the amide, to get it past the BBB suggests that NAC can't pass the BBB.
Indeed a search confirmed that.
See
http://www.fasebj.org/cgi/content/full/15/1/243
where it says,
"N-acetylcysteine does not cross the blood–brain barrier"

I have little medical knowledge, I am a chemist, and had better leave the interpretation of the paper you quote, to somone better medically quailified.

Sorry my forulae came out jumbled in my other message,
Best wishes
Ron