I don't think you need to handle it for file access outside of the pool. Windows and other apps already do that.
If Drivepool would "eject" the pool, at least in the sense that Drivepool itself wasn't holding a lock on anything on the drive, the drive(s) could then be ejected in Windows like any other drive. Ideally the Drivepool eject would pause any duplication or balancing operations for the ejected pool. I mention that because of a post I saw about a bug that was causing either Drivepool or Scanner to run unnecessary operations on a drive under certain circumstances. Maybe the eject needs to pause Scanner also?