I'm using a Clamp constraint to animate a camera along a path for a long, continuous fly-by shot with pauses and target changes along the way. I'm keyframing on the transform of the camera dolly, but every time the path is modified, particularly extended, the position of the dolly in all the previous keyframes is thrown out, requiring lengthy repositioning and re-keying to correct all the timing. I've experimented with using a Track constraint instead, and animating the Offset, but this still suffers the same issue when the path is modified.
This behaviour suggests to me that, even when keying on a transform rather than an offset, Blender is doing some math and quietly converting everything to a relative offest. When the length of the path is modified then all the previously keyed positions also change because they're relative to the length of the path rather than being, say, fixed distances from the start of the path.
Is there a way to 'lock' an object to a fixed position in a keyframe so that it doesn't move when the length of the path is extended, i.e. if object A is X units from the start of the path in keyframe Y, when segments are added to the path then object A is still X units from the start of the path in keyframe Y.
Gracious thanks for any advice, it's much appreciated and may save me from this slow descent into madness. My apologies if my description is not clear, if this question has already been solved elsewhere, but especially if I am doing something really dumb, missing a trick, or otherwise dementedly trying to stuff a round thing into a square hole.