Best practices for ember subnavs

You may want to try using a component for this. And have the component render into a named outlet in your main application template.

The nice thing is that the component can be dropped in and shared in all sorts of places, and the context (controller and models) can be passed into it making it a very flexible construct.

In handlebars the component

{{sub-nav}}

I worked out a a basic example of components here.

If you could put together a basic JSBin of the concept you are trying to achieve, I could take a stab at this.