How to make a menu that looks like a smartphone/tablet?

I was actually in the process of doing the exact same thing. I ended up using a camera, rendering it to a texture, assigning the texture on a cube and then calling Graphics.DrawTexture in OnPostRender() on that camera to draw the individual elements. It's a solution that is very similar to the built-in GUI in terms of programming, so you won't have to modify your existing code all that much. Then i raycasted at the mouse position, checked if it hit that cube and used the distance from the center of the cube to turn the world coordinates into pixel coordinates. For the rest, you'd have to program your own UI system that works with an XML parser.

Converting world coordinates to render texture coordinates, and checking whether they fit in a rect:

 function CheckRect(myRect:Rect):boolean{
 var ray=mainCam.camera.ScreenPointToRay(Input.mousePosition);
 var screenPos:Vector2;
 if(Physics.Raycast(shoeRay,hit,2f,layerMaskfaf)){
 padPointer.transform.position=hit.point; //The render texture was 2048x1024
 screenPos.x=2048*((-tabletScreen.transform.localScale.x/2)-(tabletScreen.transform.localPosition.x-padPointer.transform.localPosition.x))/-tabletScreen.transform.localScale.x;
 screenPos.y=1024*((tabletScreen.transform.localScale.y/2f)+(tabletScreen.transform.localPosition.y-padPointer.transform.localPosition.y))/tabletScreen.transform.localScale.y;
 
 //Debug.Log(screenPos);
 }
 else
 return false;
 if(myRect.Contains(screenPos))
 return true;
 return false;
 }

Drawing a texture on the tablet camera's screen space:

 function OnPostRender(){
 GL.PushMatrix();
 GL.LoadPixelMatrix();
 Graphics.DrawTexture(Rect(0,0,2048,1024),myTex,0,0,0,0,null);
 //Draw more things based on what you read from the .xml
 GL.PopMatrix();
 }

As for the rest, you can read data from an .xml parser and just parse them as screen coordinates and textures. A basic button function for example would do that, as well as checking whether the player is hovering over it with CheckRect and is clicking.

Remember, all of the pixel coordinates will actually be coordinates that correspond to the resolution of the rendertexture. Not your monitor's resolution.

Here's a screenshot of the result it produces: