Ok, this is not hard to do manually, but just a suggestion for a nifty feature.
Imagine a closed path, then you have a bone constrained to it with a path constraint. Then you have 'path position' keyed to 0 at frame 0, and to 100 at last frame. Would be cool if you could offset these. It would work only for keys that are 100 units different for obvious reasons.
What do you think?)
Cheers