I have the same question.
I know there is a 'get mouse position' action. You could possibly use this to start.
Then use the getmousedelta action to calculate the drag.
Then somehow use the mouse delta to control a physics force that will build up the further you drag.
Finally, when you lift your finger, it will launch the projectile.
Just taking a guess here, i'm very intertested in getting this working, so keep me updated if you have any luck.