I was trying to think of something that would complicate such a simple idea.
lol,,,, I do like your idea of some sort of hydraulic tensioner. Thank you for the idea, but for my Lego mind storming I'm just going off that idea and thinking of something like an idler sprocket or just a simple wheel (wheel would probably be better) on a spring loaded shaft. The spring would keep some tension on it but the shaft would stop it from giving too much. As tension would reverse from one direction to the other. Like a double pulley on a shaft with a mount in the middle, keeps tension but limits travel. Yea,yea,yea,, it sounds complicated but is quite simple with one moving part.