It would be possible to create a filter resource manager. It would own the users file space, which would be projected onto the file system. Most of the i/o calls would be passed on to the file system. The write() call would have a new failure mode, “Allocate Used Up”.
This would be a lot of work, especially if you are not familiar wit resource managers. There is some cpu overhead involved, but on a fast processor, say anything like a Pentium IV, it would probably not be very noticeable.