It may not be exactly what you want, but I use php to encrypt LimeSurvey data immediately after the "Submit" button is pressed. This technique uses LimeSurvey's "End URL" setting to direct to a link for the php script. It happens very quickly and without the user's awareness.
As for decrypting, I use Navicat to run an export script (we use MySQL).
I realize that you would like LimeSurvey to do all of this, but at least this might give you something to go on in case this feature is not added.