Can AssetPostprocessor change *defaults*, just for first import to project, but not override user settings permanently?

If I do something like this:

 public class FixFBXOptions : AssetPostprocessor
 {
     public void OnPostprocessModel(GameObject g)
     {
         // Never import materials by default.
         var modelImporter = assetImporter as ModelImporter;
         modelImporter.importMaterials = false;
     }
 }

... on adding an FBX to the project, it'll have no materials. That's fine and what I want. But if I then click 'import materials' and apply, the checkbox just gets un-checked and no materials are imported. The same thing happens using OnPreprocessModel.

What I really want is a way to run some code to set import options once, on a project member adding the asset to the project for the first time, but for that code to never run again and to let people change the options afterwards. Is that possible using AssetPostprocessor? I don't want to have to write a custom 'add asset to project' dialogue - I imagine that will end up being rather hacky.

Note: this isn't the same as when an instance of Unity sees the file for the first time, so the trick I've seen of checking if the asset database can load the file won't work.