Nothing fancy here, but for a "basic" mini-cam border all I did was add this to the minicamera object ->

#pragma strict
@script ExecuteInEditMode
 
var mySkin : GUISkin ;

function OnGUI(){
   if(mySkin)
      GUI.skin = mySkin ;
   var cam : Camera = transform.camera ;
   GUI.Box(Rect(cam.pixelRect.x, (Screen.height - cam.pixelRect.yMax), cam.pixelWidth, cam.pixelHeight), "") ;
}

Then made a gui skin for it, made the box->normal->background a 32x32 image with a transparent background and a 2px wide color-border around the edges.
Should be a decent starting point anyway, and could easily make it a little better with a "good" border image.