Chris:
This command is already available in GDL, in the 2D Script you use the
PROJECT2 projection_code, angle, method command, you can read about its variables in the GDL Reference Guide, but the most frequent use is a top down image of the 3D model using
PROJECT2 3, 270, 2. Its down side is that ArchiCAD is creating a 3D image for this symbol for each part you place, so many complicated parts can slow down the 2D Window. The faster 2D generation method is to figure out using trigonometry what the equivalent 2D symbols would be for the rotated object, but this not easy with curved objects.
The object you placed for download is missing the "barelslat" macro, so I was not able to look at your object.
David