Usually I try my best to pre-assign an Exit seat (allowed on NH), but last time I had only booked a week before departure during the summer peak so I was barely able to get the seats, let alone seat assignments.

I was flying ICN-NRT-LHR-NRT-ICN, and couldn't find a seat that I liked to pre-assign on the LHR-NRT segment. So I asked the checkin agent at NRT if she could leave a note to LHR asking for an Exit Aisle seat for the LHR-NRT flight and she happily agreed.

So I was rather pleasantly surprised when I reached LHR (rather unexpectedly late due to the "reliable" British trains->barely an hour before departure) and they told me they had indeed gotten me an Exit Aisle seat, my favourite one too!

Conclusion: If you can't get an assignment, you can still put in a request and, if you're lucky, they will fulfill it.