Well, I've just tried to assign a grayscale profile within Scribus to the images but it's not possible - I've installed grayscale ICC profiles but within Scribus I can only choose between RGB profiles; the grayscale profiles are not offered as an option.
Ok, I'll try to embed a grayscale profile into the image within GIMP. Stay tuned ;-) (No, don't do it, this will take some time...)
Thanks for your help so far!