vector (the colour) without affecting the upper vector (the outline). is there such a path tool? if not, would it be hard to implement? maybe it could be done with a script or something?
I have often wondered about something similar too, but I doubt it would be worth it. I don't know how often you do this, but duplicating an object can be done with one key press (Ctrl+D).
Another alternative, perhaps of more general use, would be a tool or script that takes the current object, duplicates it without bringing the duplicate to the front in stacking order, and turns off the stroke on the rear object and the fill on the back object. In other words, it splits a given object into a fill object and a stroke object. You could then go on to cut up either one of those, or to place objects between the fill and the stroke (something I find myself doing a lot), or to adjust the path of the stroke object so it doesn't quite fit the fill (for a sort of hand-coloured effect), or anything else you might be able to think of.
OTOH, it does strike me as a minor action that just speeds up workflow without adding any new capabilities, so it would probably be best to implement it as a script and let the user assign a key-combo to it if he wishes, rather than bloating out the core program.