A black and white adjustment layer does not change the color mode of the photo, so no, it won't be on the black plate only. Sorry.
You can (kind of) make that with blending spaces, but it´s quite difficult... different images usually needs different color and blending mode settings for Black&white "masks"... Heres example of one case...
Theres 2 black (or grey) rectangles, one behind image (black 100%/Normal blending mode), and one front (Black 70% / Overlay 100% Blending Mode) and image itself has been blended with Screen Blending Mode.... By changing percentage of black in fron rectangle and blending mode values, it´s possible to change most of the normal RGB images to grayscale... conversion is not that good, but image will print black plate only.
And here´s how it looks after black channel has been switched off...
And it goes that way to PDF as well....
Big question is of course - why bother do it hard way, if you can do it so easy in Photoshop?
That's quite interesting....
Interesting stuff! Thanks for the replies. I mostly just wondered if there was an easy way to keep all the data together. I hate having several versions of a file, but I guess it's necessary sometimes. At least until they start springing for color on everything! :-)