alight, here is a commentary, and a solution for you too look deeper for a better solution if possible.
When manually writing game code for BOX2D and other physics engines. The programmer has the ability to set vector points for the collision shape. I don't know if C2 allows for setting collision shapes during run time. However you might be able find a plugin. If not you can try to use the answer below.
As you have your mouse(or touch) start drawing. draw graphical lines with no physics. These lines are done in 10px spurts burts by startx/y when first touching and when the mouse move a distance of 11px away lock the fgx line in start a new 10px line. keep this up until the player released or some other limit is met.
when then player released the mouse, iterate through each of the 10px lines and create a new line which is a PhysicsLine and joint each pLine to the previous. Now you have a physics shape.
you might be able to get away with more than 10px. Experiment as you would like.