How to automatically adjust exposure of subject to match varying exposure of background video

The long and short of it probably is, that it's not really a good idea unless you record the actual camera values e.g. in the metadata of the clips and import it. I know of some Nuke scripts to that effect based around RED cameras that do this and maybe there would be a way to live-record such data on a laptop also, but it's definitely going to be extremely tricky. Other than that I don't think even just meddling with Exposure will be sufficient, as the process is not linear to begin with and in addition it's affected by things like the camera's focal length and physical aperture. And Rick of course has a point - as soon as the sun position changes and your greenscreen doesn't match, the illusion is ruined one way or the other, too, including other things like lightwrap/ glow and color changes e.g. on sunsets. That said, it may not be impossible to work something out using sampleImage() and a complex formula, but I doubt it's something that can be covered in five lines, meaning it's probably a project for someone with experience. In any case, sharing some reference screenshots/ sample frames might help to advise further.

Mylenium

What you are trying to do requires very careful planning and a very good understanding of the quality of light. You can simulate light wrap to make the composites more realistic, but you cannot easily change the lighting direction. There is no automatic way to turn a flat front-lit shot of someone in a greenscreen studio and make them look like they are sitting on a rock when the sun setting over their left shoulder on the edge of an aspen grove just outside of Jackson Hole Wyoming. I'm also kind of worried about your statement about capturing natural movement. If the camera is moving you will have to replicate that movement in the studio. Any camera moves will have to be almost identical to work. Also, perspective is controlled by camera position so for perspective to match you will have to be very careful about the distance between the camera and your actor and make that match the distance between the camera and any object in your nature shot that is going to be fairly close to the actor's position. 

You can do roto on various parts of the actor or use other techniques to create mattes to introduce color changes, but just clicking a button and having the lighting in an afternoon shot in the north woods, and a noon shot in the desert, and a morning shot on the shore of a lake, without looking at the footage, figuring out where the light is coming from, and matching that in the studio. That kind of planning is why you hire a visual effects supervisor and bring a truck full of lighting gear to a green screen set.