We normally prefer traditional dining, but on our last two HAL cruises we did open seating.
Like you, we also prefer to eat late. We usually will show up about 1/2 hour before they stop seating people!
On both of the last two cruises, which were 11 and 12 nights, we never had a problem with getting a table for 2. And on both cruises, the Matre'd came by and told us he would be happy to set up a standing reservation at the same time every night for the same table. We chose not to, but it was definitely an option. But you do not have to make reservations every night, you can have a standing reservation if you prefer. At least that's the way it was on the Noordam and the Eurodam.
If you tend to show up during the rush for dinner and don't have a reservation, there may be problems getting a table for 2, but talk with the Matre'd and I know they will try and accommodate any requests you may have. HAL is very good about that, which is why it's one of favorite cruise lines.