Have to run out right now, but if it is a once off operation you can set up a "file" type view and rename the images from your file structure or tags. You would need to have a mixed image and audio view and you would need to import, at least temorarily, the cover art into the library.
Not sure why you want to do this however. Before tackling the expression language, is there any specific reason to name your files (or rename them in this case)? .. if you are using the audio cover art rule, naming them all folder.jpg or cover.jpg is going to work and is pretty standard practice. Technically you don't even need the image as long as it is embedded in the file. It is better to have both the embedded art and separate cover art per folder.
I also have multi-disc recordings organized the way you do. If naming is not really important, you can insure that all cover art find its way into each sub-folder by simply selecting all the albums and Right-click =>Cover Art => Save Cover art to locations specified in options. Provided the options are set-up for audio art in each folder it will update everything. Assuming you have correctly embedded the cover art in the file first of course.