Hello,
I think a useful option for path constraint spacing would be 'proportional percent'. The spacing slider dragged in the editor would have a value of 1 correspond to 1/average-bone-length. Each bone would then be spaced along the path according to the slider setting multiplied by its own length. Thus each bone would be spaced slider-value*bone-length/average-bone-length along the path.

This would allow you to have non-uniform but proportionally-constant spacing.

I ran into the need for this when trying to make a setup for controlling armadillo body 'sections' with a path.