We are working on migrating to DNN. Our current site image-based top-level navigation.
I am working on trying to move us aways from that but may not be successful.
We have decision-makers hung up on the current typography. 
We also want to avoid font embedding until it becomes mature enough to be widely-supported.

This brings me to my question: 

Is there a way to do image-based navigation for the root-level links in the navigation component?
This child links should all be text-based as they are normally.

My primary concern with any of the font replacement solutions - including Cufon - is accessibility.

As a public entity, we are facing possible accessibility requirements.
There's considerable question in the forums on compatibility with screen readers and font replacement solutions (Cufon included).
Visually impaired readers also need to be able to control the size and color of fonts to be truly accessible.

Do you know of specific successes in this area?

"I don't see how using images would be any better"

You are absolutely right Timo.

That's why I am trying to steer us completely away from images for navigation or font replacement..

I was searching for a DNN-based image nav solution as a possible alternative if I lose the battle.