Seat assignment in Y for 2 people on 787 on ET
 

Just wondering how full the ET flights between Hong Kong and Addis would be around Christmas and New Year. 2 people travelling. Wondering if we can chance getting a window seat and the corresponding aisle seat on the window side and hope that no one would sit in the middle.....

Traditionally this is high season. Not sure about the particular route, but I would be surprised if it was a half empty flight.
ET check in agents always allocate seats from front to rear, so in order to stand a good chance of a free middle seat, make sure you choose a row in the far back of the plane. Also, do the trick where you book aisle and window. Very few people will pick a middle seat if they have another option. And in case someone gets the middle seat, chances are they'd be very happy for swap for one of your seats. It's a classic trick when traveling two people together. If one of you are Star Alliance gold, it increases the chances of a free seat next to you.

Booked a window and aisle seat right now on one of the rows in the front with the middle seat empty. Not planning to pick a row in the back since don't want to have 200 people in front of me when getting off the plane. But then again, I would actually be deplaning in Addis and going through passport control there.

HKG to ADD has been almost completely full every time I've flown it, I don't expect you'll have much of a chance. Personally I'd take the center section both aisles and hope for the best. Best case you have an empty middle and no one has to climb over for bathroom breaks, worse case you can swap with the middle person to sit together.

If one of you are Star Alliance Gold, then ET does block the seat next to that person automatically until the flight is full. But when the counter and gate agents go to assign seats, they totally ignore this. Departing Africa, I've noticed not many people seem to check in online as the seat map will be near empty the day before, but flight 100% full at departure. From HKG it may be a little different, ideally you'd want a higher percentage of people checking in online if you're *G so your seat block will stick as long as possible.