Can't help with the excess but check the fine print, some cars are allowed on dirt roads, some rental companies require you to hire a 4wd of some sort for it though. I'm fairly certain one of the "big 5" (Avis, Thrifty, Budget, Hertz and Europcar) has some line in the T&C about "cars may be driven on high quality dirt roads such as..."
Does a dirt driveway count as a dirt road?