Why not increase the number of Flight Data Recorders and add jettisoning capability?
The simplest argument to me is that no location on the aircraft is particularly special when it comes to locating black boxes in this sense. The recorders that aren't found are either in exceptionally remote locations or underwater - I don't see anything special about an engine that makes it more likely to be found than the wreckage of the fuselage.

No matter where you place them, there are going to be circumstances where they can't be found.