Eric
The only way to do this is to create a Corridor Model (can be a simple straight line alignment from A to B across the site) - the Surface Slicer has no concept of Original / Design - you can argue that each surface has a classification and therefore we can "Know", however you can also have multiple Existing and Multiple Design or Multiple Work In Progress surfaces shown in a Surface Slicer View - so we would have to have a tag in the Surfaces dialog that allowed you to Tag an Existing and a Design surface so that you could see a shaded view between two selected surfaces while still having the multiple surfaces visible.
This is an enhancement Request - I have forwarded to TBC Team to consider
Alan