The main challenge that I am seeing is how you would allow the user to create a sprite shape and then the computer will dynamically know the boundary of that shape. I know you can create any shape you want and change the bounding box to fit that shape. But that customization of the bounding box is done by the designer of the game. I'm not sure if it's possible to give the user the ability to do that on the fly.
But to answer your initial question of how to draw lines with physics behavior, it sounds like you are asking two different questions. If the question was just how do you add physic characteristics to a line, than that's easy as you would just draw a sprite that looks like a line and give it the behavior or physics. But you're probably asking more about the drawing part meaning how does a user dynamically create a sprite with a fitted bounding box and that's of course the hard part.
So to answer your question, I don't know. But I'd be interested if it can be done.