This doesn't currently exist on the public market. I can see what you're saying, and it's certainly an interesting proposition.
Currently, when we want to capture an image that uses exposure ranges too great for the camera, the normal solution for this is to use some form of HDR photography:
- Mobile phones have HDR modes built in.
- Most DSLR's have an AEB (Auto Bracketing) Mode that takes more then one photo at different exposures.
- In manual mode we can bracket photos using exposure compensation.
- In video, we shoot 'flat' to decrease contrast and give us a wider exposure range to edit with in post.
These bracketed photos can be merged using the HDR tool in Photoshop, or you could custom merge them using blend modes and masks to get the result you want.
What I'm trying to say is that we already have ways to achieve the same result as your idea, using tools that are already widely available. I'm not sure if there would be much interest for a custom tool to do what you propose as it has such a specific use case.
As per the comments on the post, it could also be detrimental to photos - if it wasn't used, will it decrease quality, darken the photo or change the refraction of light through this tool?
Another thing to think about, is that for this tool to work correctly, it would have to have a live output from the camera LCD to know exactly what parts to darken and lighten. If your tool is any distance away from the sensor, it will be difficult for it to know exactly where the light is hitting the sensor, so it will have to read this from the camera itself. And then how does it work out how much to modify light?
Might be opening a can of worms. If you've got an idea, go ahead and give it a go :) But realistically I think most photographers would stick with what they have until it becomes mainstream and/or manufacturers develop and offer it.