Jay, there's a version on the warehouse at http://sketchup.google.com/3dwarehouse/details?mid=8b02334bbd1681ac6cd8ad826e9017b8&prevstart=0 that can be fired with space.
Basically, I used two objects, making up the arrow, to get the vector for the ray. The start point uses the hidden sphere (I realise now that wasn't needed). There's a hash variable, @@thruster... so, object [1] has @@thruster[1] tied to it by name. When the ray hits an object, it retrieves the object's name (so, [1]), and activates @@thruster[1]. Inside each object is a small thruster, that always faces the gun; this is what pushes the object when @@thruster is activated. There's also an @@obj variable, which doesn't do anything (I can't remember why I thought I needed it 😄)