I have not come across a stock subassembly that allows the variable depth, but there is a solution that could automate this for you. It involves creating a "depth" profile, which would represent the depth of your pavement material over the station range that you need. Add this profile under the same parent alignment as your road design. (This will be needed in your corridor as a slope/elevation target). From there you will need to add a generic link (suggest the LinkSlope&VerticalDeflection) to your assembly baseline marker. This link is used just to incorporate the depth measurement into your assembly and will target the depth profile within your corridor. It should be added to your overall assembly as part of the same group as the pavement subassembly. Also, you will need to make sure that the generic link subassembly is "above" the pave subassembly in the Construction order. (See Image1). Next, in your Assembly properties you'll need to set a Paramter Reference for pavement material that you are using to use the End Elevation of the generic link. (See Image2). And finally, you'll need to set the region targets in your corridor so that the generic link targets the depth profile.
It seems like quite a few steps, but it will accomplish the job. One drawback to this is when you view your sections, or create section views. If you use the Automatic elevation range for section views, it will elongate your views. The quick and easy fix to that is to use a "User Defined" height.
Maybe there is an easier way, but this will work if nothing else comes your way.
