I don't know a specific route, but Touring Club Italiano publishes really good maps of Italy that show all the minor roads. Order them direct from TCI at www.touringclub.it or from www.trektools.com.

When I cycled around Italy (Tuscany & Umbria) last May I found that the really long tunnels usually had a very minor, unused road going over the top. In most cases this was the only road before the tunnel was built. I have a good LED taillight on my bike, I was really glad to have it for the one kind of long (1200m, 3/4 mile) tunnel I had to go through. In the one tunnel I went through, I turned on my blinkie light, hugged the curb, and rode the straightest line I could. Motorists were VERY courteous, including the big truck that waited behind me for oncoming traffic to clear before he passed.

I rode on a few mountain roads with roofs built over them to protect the road from avalanches, the taillight was good to have there, too.