Thanks, R0J0! I'm trying to do it with Canvas instead :))
I used a canvas layer for the baskets in:
https://www.flippeeball.com
The sides of the nets are each made up of three line-shaped sprites connected with revolute joints. The "knots" of the net are all sprites too and they're all connected up with distance joints - but distance joints are invisible so I draw between the knots using the canvas layer.
As for your problem, I think this might work: make the spring from several segments connected with revolute joints. Then have two immovable/invisible sprites - one above the end of the spring and one below. Then attach distance joints from each to the tip of the spring. Actually, I think you might need a third - to the right of the spring's tip - to hold it taut.
EDIT: Just realised that I don't think my solution will make the spring bend in a curve. Maybe you need multiple distance joints above and below. Or maybe you've solved it by drawing a curve that's not actually represented by a physics object.