If I understand correctly what you want to achieve, you are looking for the concept of "imposters". These are precisely 2D representations of 3D objects, used to decrease the geometric complexity of rendering the 3D environment.
You can think of them pretty much as pictures of the 3D objects that are used as textures of 2D objects. The canonical reference for those is probably the chapter 21 of the book GPU Gems 3, called "True Impostors". It's publicly available here. Another important reference is the chapter 5.7 of the book Game Programming Gems v.2, called "Imposters: Adding Clutter”, in Game Programming Gems 2, Tom Forsyth".
And they can evenly be generated on the fly, to deal with small changes in view-angle or with run-time generated content. Take a look at the great Gamasutra article on the topic, called "Dynamic 2D Imposters: A Simple, Efficient DirectX 9 Implementation". A further development of the concept has appeared at "Billboard Clouds for Extreme Model Simplification".
For a gentle great explanation of the concept, see this blog entry: http://blog.wolfire.com/2010/10/Imposters
Lastly, as you are using Unity, I've heard of a plugin that automatizes such process. It is called Real Imposters LOD System.
UPDATE: for the sake of completion, I thought of adding two things that I remembered of recently. First is: there is a great GDC video where SimCity's art director explains how they have made (heavy) use of imposters to achieve top-notch graphical achievements of that game: "GDC - Building SimCity: Art in the Service of Simulation".. Second, for those interested in Unreal Engine, it has a built-in feature to implement imposters, as can be seen in docs