I've just submitted some code to the tracker which displays a button on the Workshop page that says "Download all submissions to a .zip file". You can find the code in the tracker: MDL-40879
The function emulates the one in the assign module and has only been tested on powerpoint files but should work for anything. It works for moodle versions 2.4 through 2.9.
Please let me know if you find any problems with it.