This is doable, but you'd have to write a custom XML file for the installer. I could see more people wanting a mod installer that would live longer than a few months. How many XML files does a mod normally replace? Do the changed nodes have names that stay static? PM me your Layout.xml and the original Layout.xml. I'd like to diff them.
My original question didn't really get answered, so let me give you an example. If I tell the installer to install custom empty seat images and a dealer button, would the XML layout be different than if I just wanted it to install a new dealer button? If so, it'd have to have a feature to allow injecting xml changes into the layout file, which would solve the above problem as well.
I'm not allowed to moonlight, but I can write donation based software. Basically it's free and you donate whatever you think is fair. Then I'll match the year's total donation (I'm not expecting much) and give it to Child's Play. The catch being I get to deduct it from my taxes and you don't.
Keep the ideas coming, so I can finalize a functional spec.