If I understand you correctly, will such a modification allow us to use existing files on a drive without "locking" them to the mining process? If so, will that not lead to a free-for-all for all huge data centres around the world who can then mine while still hosting server space? I believe dedicating free space to plotting/mining has so far been a deterrent to those players.
You haven't quite understood (but that is okay as I have not been that explicit as I'm trying to reach out to devs that might be able to "read between the lines").
The files that would be used would need to have been pre-announced (so that their hashes are actually known).
Exactly how that should work I haven't spent that much time considering so far (as it is mostly just an idea at this stage) but it would be fairly easy to limit the rate of expansion (making it more likely that smaller operators could expand to keep up with the total storage if they wanted to).
Over time the advantage would still be with those with more (and faster) storage but in order to get the files that the entire network might consider would first require the entire network to have accepted them (the "proof" is not the file content itself but the file content being modified from a starting nonce).