I implemented the grapple gun as mentioned below and it works semi-perfectly. I discovered that joint lists are independent, whereas I assumed they'd be inter-related. Iterating through the player body only gives me the joint for the gun. For some reason, I expected to see the joint between the gun and the grappled object. Not so!
I don't limit the gun's range of motion (it runs from 0..360) and have given it a HUUGE torque. Much hilarity ensues when you grapple an object and then try to move the object through the player body. Jumps and leaps of enormous proportions!
The raycasting took a little while to figure out -- I had to break out the code for the Ray-Tracing that is in the Box2D testbed demo. First off, I had to implement this via a callback giving the start and end points of a ray projected out from the gun. Easy enough. Just have to do some rotations and transforms to get the ray to start where the gun is situated. The next part was a learning experience... I was able to grapple through walls which didn't make sense.
The reason behind this is that the callback you supply is not guaranteed to return the nearest object the ray encounters. You have to set it up to return a fractional amount as an indicator to the caller. This clips the ray to that point. If there is an object further out that hasn't been reported, it will not be reported. If there is an object closer that hasn't been reported, the callback should iterate again with that object, at which point you again return a fractional amount to clip things. Note -- this is only if you want to locate the nearest object. Other return values will get you different results.
See below for the player / gun body grappling a circle body.