As far as I am aware none of the "Southern overnights" other than the 0102 & 0402 from London Victoria will be mentioned on reservations.
If you use an Advance from the previous day then I would recommend following the itinerary, but I would be interested to know how it recommends you get across London to Victoria and what timings are allowed. All Southern-branded overnight services do have conductors, and you could always ask to extend your journey a little beyond the previous day's validity; be aware that some trains around this time, especially the 0502 Victoria-Brighton, do have thorough ticket checks at least from time to time, and people do get picked up on using previous days' tickets without permission.
So in other words, you may not be bound to a specific train with a reservation, but if you're going beyond the time overnight (0429) when your ticket expires if it's an Advance booked for the previous day, you would do best to speak to staff for advice. There will always be someone around to assist.