Well, the grapple hook idea has failed miserably. As soon as I put a chase camera in place and started trying it out, it turned out to be unplayable. Can't see where you are firing it, can't figure out where the hell you are once you start swinging around, total fail.
Oh well, never mind. I've just removed the hook and left the game as is, so I can try to think up another gimmick of some kind for a third person platformer and see how we go.
Been working on a chase camera since then, using this excellent reference:
Turns out projecting rays from the player to the corners of the near plane of the view frustrum is an excellent way to keep the player in view while stopping the camera from peeking through walls at extreme angles. Its a start, not quite playable yet, need to look at moving the camera more smoothly when it has to change position, but its getting there.