Girls on 6Annonce post their working hours, so any who stops at midnight would be out of the equation, I suppose. The others usually have 24/7 mention, which is obviously nonsense.
You might try to check after midnight (UK time) to see which girls are "online" with green light, they should be fair game. Also if you check some in the evening, they have a "late night girls" banner on top, with a scrolling list of girls - what's annoying is that I can't see any other way of checking which girl is a late-night one unless you're lucky and actually see the page of one and then have access to the listing.
You might want to contact such a girl the day before the meeting to be sure she'll be ok to meet you late at night.