I did this recently with no problems.

Use the NH PNR (or they now say the UA one will also work, I haven't tried it) to select seats.

There must be some reverse notification to UA, because the seats are showing on the UA site - I can't change seats there, but they're showing.

The only thing I've been unable to do is pre-select a meal since that seems to limited to NH members. Thtat's not a huge biggie as I'm looking forward to the Japanese meal, which I'm sure they'll have plenty of on board.