I have a similar task to complete and have been messing with this all morning, i think i am on to something good here. I am combining a few of the techniques i have seen this morning and adding my own twist. Windpig, would you mind posting either the original or a very low compression JPG version of the crop you have shown? The compression artifacts are making this harder than it should be.
I'd use my own images, but they aren't as difficult as yours being that the value differences between the hair and background are so close. I've noticed that every single tutorial uses someone with black hair on a white background.
Here goes, max quality, 100 crop.
PM me and I can get you a TIFF file.