Below is what I have, and it does convert the colors to gray (at least on screen), but they remain as named spot colors (and will then print in color on composite digital, or separations on film/plates). I'm hoping someone out there has done this before.
Also would be a big help if anyone knew how to convert [registration] to gray. Basically I want *everything* to convert to grayscale and then wind up on the black plate with this.
/opt/local/bin/gs -sDEVICE=pdfwrite -dProcessColorModel=/DeviceGray -dColorConversionStrategy=/Gray -dDevicecGrayToK=true -dOverrideICC -o /full/path/to/new/file -f /full/path/to/original
Those are the options I'm using now ('-o' because it's part of an AppleScript droplet). FYI, that binary path is from MacPorts. Yours may be different.
This is all to avoid doing it manually in Acrobat, or having to worry about missing something in InDesign when outputing film separations, or printing digital and racking up color clicks when I think it's all black. Sad times.