Hi Paul. I was super excited to see the "official" parking tutorial with some clarifications that improved my own workflow quite a bit (I'm still trying to automate a bit more based on CAD parking lines that are already provided by the client so I don't have to do the added step of creating distribution lines...but your solution was pretty easy considering the old way of doing things). The random car paint colors using Forest map was great...took me a bit more time to tweak my library, but it's worth it.
Regarding the vehicle materials, I created a preset for a standard parking lot after I got everything working following your tutorial, but when I went to apply that preset to a new project, the car materials were all messed up (metal tires, non-transparent glass, way-off paint colors), so something obviously was getting mismatched between the individual car materials and the forest material. I checked the Material slot in the Geometry rollout and everything seemed to be assigned correctly, but I could click the assigned material and see there were two instances of each in the scene (one on the Forest001 object itself, and one on a hidden object on the forest_templates layer). When I clicked "Clear" above the Geometry>Material slot, everything magically went back to the way it should be and renders correctly. However, the Material slot for each object is empty.
I don't want to mess around with it much more since it seems to be working, but does this issue make any sense to you? It's one minor extra step to assign the Forest Pack preset and then select all the Geometry items and Clear their Material, but it seems like that shouldn't be needed. Is it possible I created my preset incorrectly? I can send example files if needed.