How do I get a plane to render above other planes

Graphics Shaders & Materials
1

I have a plane that intersets with some other geometry ( its a siren light )
However I wish to render it without the intersection, ie over the top of all the other geometry.

See the attached image for what I mean.
Is there a way to do this, I tried renderer queues, and I am using a simple additive shader.

1211211--49256--$Screen Shot 2013-04-06 at 6.40.46 AM.png

2

You could do it with a second camera - clearing the zbuffer only, and having it render the plane

3

I noticed a 2-3 fps drop with a second camera, so I was trying to avoid that.