Auke,
Cheers. Eventually for expediency sake we took the 304 all the way to?? then the 201 to Chayaphum then the way you suggested to Udorn the only "bad" surface bits were 228 and the as yet not repaved section of 304 somewhere in the middle, but that is relatively short. The 304 around Koa Yai is "relatively pretty but busy.
Second day we did the Loei, Lon Sok loop and returned to Udorn last leg a bit boring but the loop is probably some of the best sport bike riding in Thailand.
Back to BKK went straight Udorn to Koa Yai and home along 305 then cut down to 304.