Can't really say specifically for the NRT-DTW route, and it is past the summer tourist rush, but I have gotten op-upped two years in a row (as a Gold) during my summer transpacs (NRT-MSP and PDX-NRT).

Being during the summer tourist rush, both these flights were packed to the gills in economy. I would guess there were fewer elites on these flights given the season, and this probably helped as well.

On the PDX-NRT segment, I arrived early at the airport and got my op-up when I checked in at the main counter. I was also given the option of being bumped, but since I already had my WBC seat assignment in hand, and really had to be back in Tokyo the next day anyway, I opted to pass.

On the NRT-MSP segment, I was pulled out of line while going through the gate and, among profuse apologies from the two GAs, given a WBC boarding pass.