I've been to Dunns River Falls, but not for a wedding (just as a plain old tourist). From what I recall, Dunns River Falls was set up like an amusement park. What I mean by this is that in order to get to the falls, you have to pay admission, receive a ticket, and walk through a gate. Then, once on the other side of the gate, there are tour guides and visitors who have paid the admission. I think there were also vendors selling water bottles, etc.
If your wedding is outside this gated area, I don't think you'll be able to "sneak" away to the falls. So you probably would want to schedule your TTD for the next day.
If your wedding is inside this gated area, then I am not sure if I can answer your question. I don't really remember the layout of the "park" or how big it is. I just remember that it wasn't an open place that you can drive up to and take a stroll like an open, playground-type of park.