This is very similar to the Power Stroke tool blueprint: https://blueprints.launchpad.net/inkscape/+spec/power-stroke
Except that for what I see in the webpage, the Illustrator plugin let's you adjust the left and right sides independently (right? the page is not very descriptive... what is the second row of spin boxes for?).
Bulia: i thought (and some other people suggested similar things on IRC) about some per-node property (or per % of path length -whatever is better) that attaches other objects to a specific node in a path. So moving the object would move the path, and moving the nod in the path would move the object. Like constructing a skeleton or ragdoll like one would do in a animation program. Not entirely related to this, but if you're going to extend the attributes of nodes maybe is a good thing to keep it in mind.
Regards.