Is there a way to change the shader on all meshes without doing it manually one by one?

Since we can't easily list all shaders, using a TextField is the best I've got, after modifing the script from wiki:

 using System;
 using UnityEditor;
 using UnityEngine;
 
 class MassMaterialEditor : EditorWindow
 {
     static MassMaterialEditor window;
     
     String oldShaderName;
     Color oldMainColor;
     Color oldSpecColor;
     float oldShininess;
     Texture2D oldLightmap;
 
     [MenuItem("Character Generator/Mass Material Editor")]
     static void Execute()
     {
         if (window == null)
             window = (MassMaterialEditor)GetWindow(typeof (MassMaterialEditor));
         window.Show();
     }
     
     void OnGUI()
     {
         Shader shader = Shader.Find(EditorGUILayout.TextField("Shader", oldShaderName));
         if (shader != null) if (shader.name != oldShaderName)
         {
             oldShaderName = shader.name;
             SetProperty("_Shader", shader);
         }
 
         EditorGUILayout.Separator();
         GUILayout.Label("--- Render Settings ---");
        
         RenderSettings.fog = EditorGUILayout.Toggle("Fog", RenderSettings.fog);
         RenderSettings.fogColor = EditorGUILayout.ColorField("Fog Color", RenderSettings.fogColor, GUILayout.Width(140));
         RenderSettings.fogDensity = EditorGUILayout.Slider("Fog Density", RenderSettings.fogDensity, 0, 1);
         RenderSettings.ambientLight = EditorGUILayout.ColorField("Ambient", RenderSettings.ambientLight, GUILayout.Width(140));
 
         EditorGUILayout.Separator();
         GUILayout.Label("--- Material Settings ---");
         GUILayout.Label("Selected Materials are modified");
 
         Color mainColor = EditorGUILayout.ColorField("Main Color", oldMainColor, GUILayout.Width(140));
         if (mainColor != oldMainColor)
         {
             oldMainColor = mainColor;
             SetProperty("_Color", mainColor);
         }
 
         Color specColor = EditorGUILayout.ColorField("Spec Color", oldSpecColor, GUILayout.Width(140));
         if (specColor != oldSpecColor)
         {
             oldSpecColor = specColor;
             SetProperty("_SpecColor", specColor);
         }
 
         float shininess = EditorGUILayout.Slider("Shininess", oldShininess, .01f, 1, GUILayout.Width(250));
         if (shininess != oldShininess)
         {
             oldShininess = shininess;
             SetProperty("_Shininess", shininess);
         }
 
         Texture2D lightmap = (Texture2D)EditorGUILayout.ObjectField("Lightmap", oldLightmap, typeof(Texture2D));
         if (lightmap != oldLightmap)
         {
             oldLightmap = lightmap;
             SetProperty("_LightMap", lightmap);
         }
     }
 
     static void SetProperty(string prop, object value)
     {
         foreach (Material m in Selection.GetFiltered(typeof(Material), SelectionMode.DeepAssets))
         {
             if (value is Shader)
             {
                 m.shader = (Shader)value;
                 continue;
             }
             
             if (!m.HasProperty(prop)) continue;
 
             if (value is float)
             {
                 m.SetFloat(prop, (float)value);
                 continue;
             }
             if (value is Color)
             {
                 m.SetColor(prop, (Color)value);
                 continue;
             }
             if (value is Texture)
             {
                 m.SetTexture(prop, (Texture)value);
                 continue;
             }
             throw new Exception("Unexpected type for " + prop + ": " + value.GetType());
         }
     }
 }

Well... for starters, I'm sure you could write an Editor script to do it for you. Here's the class overview for Editor scripts:

http://unity3d.com/support/documentation/ScriptReference/20_class_hierarchy.Editor_Classes.html

Unfortunately, Unity doesn't support multi-select editing (yet), so if you can't find a solution, you may just have to suck it up, throw on some music, and do it manually.

You could also try exporting some other way from Maya... instead of using the Maya format, you could try .FBX (or the other way around). Unity may import the materials differently using different file formats.