FWIW NRT-BKK loads (on UA) are off quite a bit for April (and beyond), so a bump might not be in the cards. My NRT-BKK flight this week was less than 50% of capacity in all classes of service, an oddity in my experience, but acknowledged by some of the FA's as a recent phenomenon. Perhaps this is due to the current politcal climate or some sort of low season?

There are hundreds of people greeting the UA flight, and most of the counters are staffed, although I did not take notice of the hotel desk specifically this week upon arrival.

You could make a reservation at a nearby hotel, without financial commitment, for the night of your possible arrival, and then venture to Hua Hin the next day. Otherwise you'll be negotiating with a metered taxi for the trip to Hua Hin. Three + hours, closer to 4,000 Baht I'd imagine. Who knows how long this driver has been working? When he stops for LPG buy him several M-150's.