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.
The administrator has disabled public write access.