Make 3d model arm rotate with camera?? How to animate a particular bone/armature using script (eg. by rotation of the cameras Y Axis)

Hi all, really enjoying using Unity and have made some pretty fun, yet also terrible games already :)

I'm trying to step up my act a little. I've created a couple of human like characters that are very well rigged. My animations using these rigs are pretty bad for now but since they only took about an hour or two in the Action Editor of Blender I'm satisfied with how it looks for now. (ie. the running, walking, jumping, idle and sitting down all look half decent).

My game I'm trying to make , you can get onto a moped and drive it about, you can also free-look using the mouse (with RMB pressed down).

MY PROBLEM IS: I want him to be able to shoot stuff whilst driving. I pretty much have this covered, except heres the hard part... I want to have the right arm of the player rotate so that it is always pointing in the same direction as the camera. My 3d model is made so that when he is in "sitting on moped position" his right arm is dead-straight and pointing at the ground

I've seen in many games I have played that this can be done, I've seen it done very well in AAA games but I can also remember many early attempts at this in the dawning age of 3D gaming and that's basically what I'm shooting for right now (i do not care if it looks terrible , or distorts the mesh, etc , at the moment, I'm more interested in how one would approach coding this functionality.

Any help is massively appreciated as always . thanks.