A bit of fair warning, the $ignorez flag will not work completely. It only renders on top of everything else if it is in the PVS to begin with. When a map is properly optimized the PVS is fairly small, and it won't render over very much or very far before it is dropped from consideration.
and before you think of it, putting it in the 3D sky won't work either, I tried that once. 3D sky components will always render behind the world no matter what.
The outline effect might be a better choice to look into. Probably by parenting an outlined object to the thing you want to track. Plus, then it won't render when you have line-of-sight to the target and get in the way, since it was coded for this kind of stuff.