Hmmm...
Making it freewheel at both ends is a much more difficult problem. Something I'd really need to think about. You could grind down the threads at each end so the screw would just spin but you'd need some sort of spring arrangement (or something) to re-engage the thread when you wanted to move in the other direction. Looking for a solution I did come across this which I know is different altogether but you might be able to adapt the concept.
https://www.youtube.com/watch?v=5xZKDlhF_os
It's an interesting problem. I'll keep thinking about it...