I'm not convinced a jettisonable recorder is worth the expense and effort. But it seems to me the FBW computer would know when the aircraft was no longer recoverable, or at least when it was no longer in controlled flight. It might activate prematurely on occasion, but data on what caused the original upset would be a lot better than no data at all. It shouldn't be that hard to set up the recorder to also jettison if the computer stopped working altogether.
Or you could just mount the recorder in Monty Python's tomato.
