this route looks fun. I have been eying a route slightly south of this that starts in Yamhill and ends at the trask river road. This looks like it would be a great route back to the valley. Oh and mtb tires are not needed for dirt and gravel. just watch any cyclocross race this next fall. A larger volume tire makes the ride more comfortable but as long as you have sufficient tire pressure so as to not pinch flat for your given tire width you will be fine. I do most of my non paved road riding on my surly long haul trucker with 1.5" schwalbe marathons and they work just fine. Figure this trip to be a two day tour with the possibility of a third day if the going is harder than anticipated and pack accordingly. This sounds less risky than other backcountry trips that i have done. Oh and if there is a road it is not backcountry.
Now all i need is a weekend of dry weather and a weekend off of work. Thanks for the idea.