what we do in our plugin jevero is to track some lines and the users can build dependancies on top of the aforementioned lines. These depenancies will generate the footwear pattern that then will be used for building the shoe.
What happens at the moment is that once a line is changed all the objects dependant on that line are updated with the new curve geometry. Say that you have a number of stabs that follow a curve and you change the curve, the stab position will be adjusted so that they follow the new geometry.
What the users requested is that before the change happens, during the edit of the curve, we preview the new position of the stabs (or anything else). To achieve this we need to have access to the preview geometry.
I hope this explains what we want to achieve. If not please let me know and I will try to clarify further the issue.