We've done something similar in the past, in that we've got a repository plugin that moves old files from moodledata to a seperate directory (the idea being that this could be an archive on a separate partition/network share), leaving a reference in the file's record in Moodle. When the file is accessed through Moodle's file API, it's then copied back into moodledata and served from there.
I'm not sure you'll find another way of doing something like this through Moodle's APIs, perhaps looking at the code of some repository plugins could give you some inspiration.
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mform not accessible from save_settings function
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