The QF forum reports that you can get into A on the 380. I wouldn't say it is easy or common, but it's not like SQ and the *A where it is literally impossible. Furthermore, the online tool does immediately pull availability from the fare bucket, as long as you go in through the "Plan and Book" tool and not simply the "plan" tool. When you go in through Plan and Book, the tool pulls real-time A-bucket availability if you tell it you're traveling in first. I know this first hand because I had a CX segment that I could not get to show up in the Plan and Book, but would show up in the Plan, and after checking EF it was because there was no A availability. So, as long as the OP is using the Plan and Book part of the tool, and they are getting the 380 segments to show up (and check the fine print that it is showing as A bucket, not D), then it is in fact available.

To the OP: just remember the 380 doesn't fly all these routes all days, make sure you're actually on an A380 day, not a 744 day.

As for how one has a seat assignment, that's a good question as I have no idea.