Well, that's kind of what I would do. The balloon could be the thruster, like they have on rockets. So you'd just want to pinch the end of it until you were ready to send it across the string. I'd probably just tape a paper clip to the opposite side from the straw and attach the clothes pin to that, since the clothes pin has to go across the distance.
Possibly both. I have an idea for it though. Like put the straw through the rope, tape it to the balloon, but then that's all I got. I don't know what to do with the clothespin or how to get it to the other side. =/