http://www.deviantart.com/deviation/11813974/
then open up the files, select the gridded square, and EDIT > CREATE PATTERN or whatever it is called.
What you say could also work, the rasterization filter, and I'm sure someone's coded something like that in, but it's not bound to work in print format, because of the ugly moire effect resized patterns that are usually created.
Take note to only use these tones in 600 dpi final scans, which will not be resized later, that's the whole point.
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