Convert Colors profile to threshold to black? [A9]

Question

Is there a reasonable way to create a color profile that can be used with Convert Colors to apply a threshold (180/255) function to convert an RGB PDF into a black-and-white PDF? Acrobat 9 (CS5), OSX 10.6.7.
I have some multipage PDF files I want to "sharpen" (make clearer) before printing, and found that opening a page in Photoshop and applying Image > Adjustment > Threshold to 180 (out of 255) does a nice job. This is a pain for multipage PDFs because I have to extract the pages from Acrobat, apply a batch action in Photoshop, and then (potentially) re-integrate them into a PDF file.
Acrobat can apply ICC profiles to convert an image via Advanced > Print Production > Convert Colors. But how can I generate an ICC profile that does something analagous to the threshold function?
One crummy way is in Photoshop. Edit > Color Settings, which allows you to define a custom CMYK profile. Under Dot gain, you can set transfer function curves for C,M,Y, and K. I tried doing that, with curves at zero up until 70%, at which point I ran them straight up to 100% with a very steep (almost vertical slope).Tried a bunch of GCR/UCR/Black conversion settings. I saved the profile so Acrobat could see it. This gave me weird results:
In Photoshop, it came close to doing what I wanted, but not totally, when I converted the image to that profile. But it wasn't drastically wrong. But when I applied the same profile in acrobat (with Convert Colors), it looked much much worse, with large blocky pixel groups and the document, which was mostly a raster image of text, almost unreadable.
(Sorry for the lack of screenshots/images -- they're at the office, I'll post them tomorrow...)
Any tips? Is there a better way to do what I want and apply a simple transform to a PDF file?
A better tool to create ICC profiles for Acrobat's Convert Colors?
Am I just using the wrong settings in Photoshop?
Thanks!

Answer 1

OK, some more info. So, my PDF contains letter-size pages with 300dpi 8bpp images that are DeviceRGB DCT-encoded (JPEG). Here's how Acrobat displays one of the characters:
(All those obvious JPEG artifacts which explain why it prints badly and would benefit from processing). It looks pretty much the same in Photoshop (via Edit Image from Acrobat, or opening the PDF file directly). Thresholding it to 180 in Photoshop does a great job:
For simplicity, I tried to build a Gray profile that did what I wanted. But it looks like Photoshop's gray profiles only let you adjust dot gain and gamma, and neither of those are sufficient to achieve this kind of effect. RGB doesn't let you use curves. So I converted the image to CMYK, and then Edit > Convert to Profile, then choose CMYK and Custom CMYK and define a profile like this:
with these Curves:
And it seems to do the right thing when the image is converted to it in Photoshop; not perfect but much better than the source:
So, back to Acrobat, and apply the profile with Convert Colors:
basically a disaster. and some very faint jpeg artifacts turn into a big blue rectangle.
So what to do? Is there a better way to construct these profiles? Or should I give up and use batch operations?