I love your work Rob! Summer's really coming through for us now isn't it? - NE wind and southerly swell.
Sorry, I've tried straightening video from line mounted cameras. All motion detection software tries to match small (say 20x20 pixels) rectangles from one frame to the next, but because of the large distortion caused by the camera movement during the recording of each frame, it can't do it. (If the distortion stayed the same, we'd be ok)
One thought I had was that the spinning is caused by the offset weight of the camera, so it might be worth mounting a similar weight to the camera on the other side of the rig. It obviously adds more weight, so more up and down movement (which isn't much problem), but maybe no swinging.
I'd try it, but the seal on my gopro failed finally and it's no more! (Reason to buy a HD one?). I did try a bicycle seat mount on the end of the bar - it's a bit too close, but interesting for short periods. Here's a couple of frames...
Here's a couple of brave last video frames...