by Paul Charsley.
Hi,
One of the web service functions that Damyon is referring to is mod_assign_export_submissions (See MDL-31886). This function will accept an assignment id as a parameter and will download a zip file containing submission files. I believe this should meet your requirements and we are hopeful of including it in Moodle 2.5.
The issue is currently blocked by MDL-32336 which is our proposed change to support file downloads in web service function.
Paul