Yes, I did it a few years ago. Took the main 210 from Udonthani to Loei, nothing spectacular. On passing Loei and having plenty of time, I decided to take the 'scenic' route to Nan. So I turned off at Phu Ruea and went that way. Sorry, can't remember the road numbers as was using an old road atlas. The idea was to follow as closely as I could to the Thai/Laos border for as long as I could.
Would I take that route again, probably not. Small isolated roads, some in extremely bad condition, landslides, no other traffic (climbed over a mountain and back down), saw no one except for one guy tending his herd and no fuel stops.
Good for an adventure to do once, maybe good for a bunch of bikers (on off-road/adventure bikes) but in my case, driving a Vigo, never again. Ended up emerging just south of Nan.