How to control render order of viewports?

I am trying to create an engineering drawing where there are several viewports on a virtual drawing sheet, each looking with a different camera at a model scene. The general approach of using renderer.setViewport works well for the views, but we need to overlay on top of this annotations (dimensions, notes, etc). The problem is, I can’t figure out a way to put anything in front of the views. Here is an image.

The diagonal line represents an annotation. It should be in front of the 3 blue-background views. I have displaced one of the views from normal to show how one covers the other. What I need to do is "push’ the viewports all the way to the back. But I cannot think of any way to do this. renderOrder doesn’t have an effect; I suspect because these are 4 separate renders. If I render the main sheet first, none of the viewports show up.

Any ideas, pointers to examples, or wild-ass guesses on how to do this?