Creating Context Menu Item for FBX files

I would like to add a context menu option for imported .FBX files. I previously had the same functionality in the AssetImporter, but I don't want this functionality to happen for all .FBXs imported. I don't want it to be automatic.

The specific functionality is just that I want to set the .FBX's animation clips' masks to 'create from this model' and set the avatar mask to every child bone.

Here's the method I used in the Asset Importer:

 /// <summary>
 /// Automatically set this model importer's mask type to 'CreateFromThisModel' and set the mask
 /// to the entire hierarchy.
 /// </summary>
 public static bool CreateMaskFromThisModel(this ModelImporter modelImporter, GameObject model)
 {
     bool changed = false;
 
     ModelImporterClipAnimation[] clips = modelImporter.clipAnimations;
 
     if (clips == null || clips.Length == 0)
         clips = modelImporter.defaultClipAnimations;
 
     ModelImporterClipAnimation[] newAnims = new ModelImporterClipAnimation[clips.Length];
 
     for (int i = 0; i < clips.Length; i++)
     {
         if (clips[i].maskType != ClipAnimationMaskType.CreateFromThisModel)
         {
             newAnims[i] = clips[i];
             newAnims[i].maskType = ClipAnimationMaskType.CreateFromThisModel;
 
             changed = true;
         }
     }
 
     AvatarMask mask = new AvatarMask();
     mask.AddTransformPath(model.transform, true);
 
     for (int i = 0; i < clips.Length; i++)
     {
         if (clips[i].maskSource == null || clips[i].maskSource.transformCount != mask.transformCount)
         {
             clips[i].ConfigureClipFromMask(mask);
             changed = true;
         }
     }
 
     modelImporter.clipAnimations = clips;
 
     return changed;
 }

This works. However, I seem to have no way to access the model importer after the import, at will. This is as far as I've gotten for my context menu.

 [MenuItem("Assets/Create Mask From This Model", isValidateFunction: true)]
 private static bool CheckIfFBX()
 {
     if (Selection.gameObjects.Length == 0)
         return false;
 
     for (int i = 0; i < Selection.gameObjects.Length; i++)
         if (!AssetDatabase.GetAssetPath(Selection.gameObjects[i]).ToLowerInvariant().EndsWith(".fbx"))
             return false;
 
     return true;
 }
 
 [MenuItem("Assets/Create Mask From This Model")]
 private static void CreateMaskFromThisModel()
 {
     for (int i = 0; i < Selection.gameObjects.Length; i++)
     {
         // get importer, clips, avatar mask, anything?
     }
 }