Jeff, Donna, and Bek have said most of what I was going to say about high-powered headlights and good visibility gear (if you haven't already got all that stuff). One more note, though: a helmet-mounted light might be best for the situation you describe. When an oncoming car starts to pull into your lane, looking straight at them with a helmet-mounted HID might get their attention and remind them you're there.

One other thing you could consider -- especially if it turns out to be the same set of idiots each night trying to pass unsafely -- is mounting a video camera to catch their plates and to show the cops exactly what they did. If your local police department takes their jobs seriously, then you might be able to rein in the worst repeat offenders.

Since no practical alternate route exists, it sounds like you're stuck with that road. I hope the extra lights help make it a manageable commute. It sounds as if switching to a car might still leave you in just as much danger...are there many motor vehicle accidents on that stretch? (There is one and only one road near where I live that I would never advise riding on despite its being bike-legal...and it's a notoriously dangerous road in a car as well.)