I went back to look at Dan's directions for doing this sharpening. They are:
Here's the procedure, which of course should be reduced to an Action to save having to do
it over and over.
1) Copy the RGB image.
2) With the copy, Convert to Profile>Custom CMYK.
3) Fill in: Heavy GCR, 70% black ink limit, 340% total ink. Dot gain is basically not relevant
as you can always lighten or darken the mask after applying it, but I just use the default
20%.
4) Click OK twice to generate the false separation.
5) Command-4 to expose the black channel, and Mode: Grayscale to discard the CMY
channels.
6) Invert the channel with Command-I, yielding a negative image.
7) Auto Levels.
Gaussian Blur, radius 2.0 pixels to eliminate noise and make for a softer sharpen.
9) Return to the RGB image and create a duplicate layer. Sharpen conventionally with a
very heavy hand--500%, 1.2 pixel Radius, 3 Threshold might be a good starting point for
most images.
10) Add a layer mask. To it, load the artificial black channel that was made in steps 1-8.
This should confine the sharpening to the desired areas.
11) If you feel the image is not sharp enough, apply a curve to the mask to lighten its
midpoint. If you find the image to be too sharp, darken the mask in the same way.
So I did this, exactly as written (no luminosity mode change), and I ended up with a file that looks...fine. Certainly workable. Was too sharp, but I darkened the mask and it seemed much better. I'm not sure how I feel about the print, but I can say that this technique does not belong in some crazy sideshow; it could be viable under some circumstances.
I have no idea how large a file to attach, but here goes.