The key will be to let them know before hand, especially if you've seen them a few times before.
Occasionally I used to go to one place 30-60 min before the official morning start, but it was someone I had seen numerous times and called in advance.
My impression is that a number of the open 24 hour ads are due to ad writers that want anyone to respond, not because the women actually work at 3 or 4 in the morning. I've never tried going then, but I don't imagine it would get a good session.