i tested my app in Android 5.1 , it doesn't work.
i read the logcat i found FileNotFoundException . the file is like "/mnt/sdcard/myFolderName/myFileName.png" (pseudo names)
i don't know why, since:
1) the files are there and they can be opened by a image viewer.
2) the app runs on Android 4.2.2 and 4.4.2.

i tried creating a smaller framebuffer , the app just couldn't start.
so your suggested way may be the only way.

the way i judge specular effect is that when viewed from different angles, a polygon has different light intensity. that is what "diffuse" lighting can't have.

i have some more questions about RTT.
rtt is only stored in vram, can i call unloadTexture on a rtt texture?
can a rtt texture be freed in order to save vram?
if a rtt is freed from vram, will calling setRenderTarget re-create the rtt ?
if vram is full, what will happen if one more texture is needed for render?

i found if i draw object3d in rtt (with World,Camera), then blitting in rtt had wrong size - much larger.
my rtt texture size is 128x32.

i think using an empty world to reset fov is good enough. just that more operations are done than simply changing the frustum. if cpu speed is high enough, it's ok.

my desktop app has noticeable specular effect.
my desktop app uses hardware GL, so max lightmul is 1 , right?
i didn't set lightmul in it.

this is test code, you'll see a white square on top left corner and it moves a little.
if your screen has high resolution, maybe it's less noticeable, test screen is 1024x600.

package abc.def;
import com.threed.jpct.*;
import javax.microedition.khronos.egl.EGLConfig;
import javax.microedition.khronos.egl.EGLDisplay;
import javax.microedition.khronos.opengles.GL10;
import android.opengl.GLSurfaceView;
import android.os.Bundle;
import android.app.Activity;

public class Sim extends Activity {
   static GLSurfaceView GLview;
   static MyRenderer renderer;
   static FrameBuffer fb;
   static World world=new World();
   static Camera cam=world.getCamera();
   static Texture tex;
   static float fov=0.1f;

   protected void onCreate(Bundle a) {
      super.onCreate(a);
      renderer=new MyRenderer();
      GLview=new GLSurfaceView(getApplication());
      GLview.setEGLContextClientVersion(2);
      GLview.setRenderer(renderer);
      setContentView(GLview); }
   @Override
   protected void onPause() { super.onPause(); GLview.onPause(); }
   @Override
   protected void onResume() { super.onResume(); GLview.onResume(); }
   @Override
   protected void onStop() { super.onStop(); }

   class MyRenderer implements GLSurfaceView.Renderer {
      public void onSurfaceCreated(GL10 g,EGLConfig c) {}
      public void onSurfaceChanged(GL10 g,int w,int h) {
         if (fb!=null) fb.dispose();
         fb=new FrameBuffer(w,h);
         cam.setFOVLimits(0.0001f,10f);
         cam.setClippingPlanes(0.1f,100f);
         tex=new Texture(1,1,new RGBColor(200,200,200));
         TextureManager.getInstance().addTexture("tex",tex);
      }
      public void onDrawFrame(GL10 g) {
         fb.clear(); cam.setFOV(fov);
         fov-=0.001f; if (fov<0.01) fov=0.1f;
         world.renderScene(fb); world.draw(fb);
         fb.blit(tex,0,0,10,10,0,0,80,80,-1,false);
         fb.display(); }
   }
}