I think the idea is great, but you may have a hard time finding a company that wants to produce such a product. Right now, with issue 94 of Dungeon/Polyhedron Magazine, Gamma World comes to D20 with Omega World. That covers mutations and technology in a post apocalyptic setting. Also, in issue 93, rules for tricked out cars were introduced.
Official, non-open sources rules aside, you can find open source rules for cars in Spycraft (and D20 modern I'm assuming), and mutations and post apocalyptic stuff in Darwin's World, which has been released as a PDF, but a printed version will soon be available.
The only thing that is really lacking in the material that's out there thus far is a detailed campaign world. Of course people just kind of assume that a GM will adapt the real world to a post apocalyptic environment, but I personally would buy a guidebook that (1) states which rules set its using as the foundation (Darwin's World, D20 modern, etc), and details much of the U.S. or even the rest of the world.
Just some thoughts off the top of my head. Feel free to submit a proposal to us; if we like it enough, we might be persuaded to pick up such a project. Contact me at
darrin_drader@hotmail.com if that sounds like something you would be interested in. Also, if interested, please list any previous publishing experience you might have.