Align and Distribute operations in Illustrator support picking a key object, by clicking on of the objects in a selection, so it get a thicker edge. This object stays in place, while every other in a selection aligns or distributes relative to it.

Sadly, this 'key object' paradigm is largely unloved in other areas, but it's also neglect in new Objects on Paths construct.
While it does not support setting a target path with it (requested here: http://illustrator.uservoice.com/forums/333657/suggestions/48988013), it also does not allow to 'fix' one object on path as adjust gaps relative to this fixated object.

What we have now is an ability to hold Opt/Alt, to scale gaps relative to the center of the array... but with key object support it would be even cooler!

Original idea by Ton Frederiks