How to auto rotate object on FBX import?

This will do it.. HOWEVER this probably isn't the right way to do it - Please read below.

 using UnityEngine;
 using UnityEditor;
 using System.Collections;
 
 public class FBXPostProcess : AssetPostprocessor
 {
     void OnPostprocessModel (GameObject g)
     {
         if(assetPath.EndsWith(".fbx"))
             Apply(g.transform);
     }
 
     void Apply (Transform transform)
     {
         transform.Rotate(new Vector3(0.0f, 180.0f, 0.0f));
     }
 }

Honestly I would suggest creating a script in you're tool that rotates 180 and bakes the transform before export.. like this one for Maya:

 $selection = ls("-selection");
 setAttr ".rotateY" 180;
 makeIdentity -apply true -t 0 -r 1 -s 0 -n 0;
 file -force -options "v=0" -typ "FBX export" -pr -es "C:/testModel.fbx";
 undo;
 undo;
 select($selection);