Robert, I don't work with this much, but I find that the 'desat' brush 'works' when brushing across the mid-range tones, but not in the split-tone highlights/shadows. Is that what you're seeing?
You might see something similar, if you try to globally desaturate a photo, and then paint back in some local saturation with the brush, you know, the b/w bride with the color bouquet cliche. This doesn't work in the current Lr implementation, you have to apply both the de-sat and re-sat with the local brush. Perhaps you're seeing a different symptom of the same issue.
I don't know if there's a workaround to achieve what you want. Maybe some of our more sophisticated artist types have an idea. (I'm more a snapshot kind of guy)
I remember reading a very long back and forth discussion involving Mark Hamburg (who played a large role in implementing the local adjustments) back in the early days, that local/global saturation presented a special mathematical problem, because any increase from zero was effectively infinite, which clearly is not right. I never did fully understand the implementation decision, but it ended up as you see.
