thanks for the reply. actually, i don't need to know how to retouch images, i know that already, i was just looking for a way to streamline a rather particular type of workflow, but i suspect that there's no point in thinking too hard about it because i doubt i will find this particular situation too often.
one comment, depending on the images, you may not want to reduce them to grayscale if you will also make a 1200 imp version ; in the case of Harry's Austen series for example it would be a great injustice not to leave the full-color illustrations in the imp 1200 version. it's only important to shift them to greyscale if you plan to really do some work on the image, like on the levels / contrast / etc. which will definitely be affected by the color mode. in which case you may want to do things right and have two sets of images, if you plan to make both versions. i didn't want to take things so far in this particular case.