What software and Version are you running. InRoadsSS2 you could draw the plan graphics then import them into a DTM as a feature and have the feature draped to the Existing Ground, then you have 3D features that can be used as point controls from the modeler.
If the plan graphical are not draped you could have the end condition seek the plan graphics as end condition target method then use a point constraints to project the point up or down to the Existing surface.
There are similar methods in InRoadsSS3 that would involve plan graphics being added to a surface terrain model, you can still use the InRoads Drap commands and set elevation tools to make the Terrain Model elements 3D, or again do an end condition target to the elements with a point control that projects to the existing surface.
These features in a DTM or Terrain should function similarly to the methods you explained using Geometry. Targets can be features or geometry. There is also no reason why you cannot have discontinuous horizontal geometry at least in InRoads, you may have a strange looking profile. If you draw the plan graphics then import them into geometry with the create vertical geometry toggle on and the method being drape to the existing surface you will get 3D geometry that can be used just as a 3D feature would be used.
