Steve, If I was looking for confirmed problems with Romex I would say you could bury it. I see it quite often buried and working fine but "it works" is not the basis for the code.
For that matter I can make a very good case that smurf tube "works" fine outside. I have a piece that sleeves the steering cable on my boat and has held up in the Florida sun for over a decade.
I don't believe "fill" will generally be the issue either. It is just whether RX is listed for wet locations and the answer is no.
