Instead of bouncing light underneath your subject, this method could make life easier.
Rubidium Wu, who goes by Crimson Engine, demonstrates the possibilities of attaching a light to your matte box. It’s a cherry on top of some already good lighting. The beauty of this comes in the convenience and the ability to move the camera along with the extra light. For some, this could be a run and gun solution for documentary work also. For him, it's an alternative to a much bulkier studio setup.
What Is an Uplight and Why Does It Work?
Lighting a subject with an uplight isn’t a new idea. Sometimes, it could be a tungsten light sitting on C-stand legs or just a white card to bounce light back into the subject's face. In the case of Wu, he seems to use a tube light under his camera to fill in his face. It helps flatten out the shadows across the face. I find that it needs to be subtle. Too much light and you’ll be introducing a horror vibe.
I think this setup is a really smart idea. My only worry with this “electrified matte box” is the quality of the LEDs being used. I would personally feel that skin tones are too important to risk using an inaccurate light. If you can pick up a used Westcott 10 x 3" flexible light mat (they’re discontinued) it might be the perfect accessory. If you wind up using a light like Wu’s, then some ND film will help dim it and avoid flicker. That way, you’re less likely to see any flickering when electronically dimming.
What's the point of the matte box if you PUT a light source inside it?
From when I can see here, the light isn't shining into the lens.