AutoCAD Civil 3D :: Subassembly - Variable Earthwork Slopes Beyond Alignments?

I just want to start by saying I have been extremely impressed with the speed and effectiveness of finding Civil 3D solutions through this forum. However I do not think there is a way to solve the problem below without making a custom sub assembly.

There is a common problem that I run into when modelling earthworks on airfields. All Taxiways and Runways will have sepirate areas that determine the slope of earthworks in that area. As an example I will explain my problem for a Runway.

Any ruwnay will have a Strip area (150m offset from its centreline) and a Clear and Graded area (105m offset from its centreline). Therefore any earthwork slopes between the runway pavement edge and 105m offset from the centreline will be determined by the requirements within the Clear and Graded Area. Likewise any earthwork slopes between 105m and 150m offsets from the centreline will be determined by the requirements of the Strip area.

My problem is that when overlaying a runway, the new earthworks required may extend beyond the clear and graded area (and into the strip) at some points but not in others.

Therefore I would like a sub assembly that does earthworks at a set slope unless it passes the clear and graded line where it would then continue the earthworks using a different slope. It should be pointed out that the clear and graded line will be present along the entire length of the runway and the earthworks may or may not extend beyond it.

Just for the sake of clarity, when I say earthworks, I mean the subassembly within autocad that will "Cut or Fill" at a set slope until it reaches a chosen surface level. Similarly the "clear and graded line" and "strip line" are 2D and mearly delineate the areas in plan.

save time and make my model more dynamic. As it stands I have been getting around this problem by modelling the earthworks at the initial slope (clear and graded area) and then manually amending the corridor to use a different assembly where the earthworks extend beyond the clear and graded line.