Get the wall geometry you want to project as solids and union them.

Get the location of the elevation you want to use for your detail curves.

Get the bounding box of all the walls

Get the length from the bounding box’s min point to the max point.

Build a cuboid which is 2x the bounding box length in each axis, and orient it so one face is on the elevation’s plane, perpendicular to the view plane.

Use a Solid.ProjectInputOnto to pull the wall geometry onto the base surface.

Extrude the resulting curves on the view’s direction, and union the surfaces into a single polysurface.

Split the face of the cuboid which received the projected geometry.

Take the outtermost surface (has an edge that was in the non-split geometry) and get it’s perimeter curves.

Remove the outtermost perimeter curves form the collection.

Draw a detail line with each of the remaining curves.

This is going to be ugly, so be certain you want to do it.