There are many tools in Slicer that allows you to do something like this.
For example, you can use Surface cut effect (in SegmentEditorExtraEffects extension) for defining a smooth cutting surface.
You can also use Baffle planner module (in SlicerHeart extension) to create a cutting surface that you can import into a segmentation (by drag-and-dropping the model into the segmentation).
It could be nice to have a combination of Draw+Scissors effect. Would you be interested in implementing it? We can help you getting started.