Great write-up Alijee!
I live in the Eugene area, no FB yet. There are many points of entrance onto the nothern interior dunes.
If you have a 4X4 just about any spur off a main road should take you there. Air your tires down a little if you like, go slow, and you will be fine if you refrain from spinning your wheels. My two sons used to ride their Honda DBs on the dunes, trailered them out with a Durango, in 2wd if I remember correctly. Oh yeah, don't forget the permit.
One word of caution, it is easy to loose your perspective of your put-in point if you fail to take propper note upon arrival, and remain aware of your surroundings as you ride about.
If you can manage a stay at Honeyman Park (not a bad place, by the way) there are trails onto the dunes right out of the campgrounds.
Great to hear that the Dunes are becoming more in use by fat bikers, BSV, Ward and now yourself. If my ship ever comes in, and a few more are discovered, we could give Scotland a run for their money.