I have flown back from Heathrow to Harbour 3 times this year on the last flight of the day, on each occasion we have landed after 11pm. The cabin crew I was chatting to said that as long as we were back in Belfast before midnight then there wasn't a problem, there were a few passengers concerned that we would end up in Aldergrove with the hassle of their cars being in Harbour carpark! Not sure how that would work, I imagine that British Airways would be responsible to get you back to Belfast to collect your car.
Was the curfew abolished along with the seats for sale a few months ago maybe?