We have surge brakes and backing up CAN be done, depending on the circumstances, for a few feet anyway. However, I don't recommend it unless you REALLY know what you are doing as it is very easy to put a lot of stress on the toad and the dolly if things are not perfectly aligned. Most of the time that I have wanted to back up, I couldn't (pulling into a gas station, going down a dead end road, etc.) so we just unhooked and did what needed to be done.
Even with a toad 4 down, you really shouldn't back up. The tow bars aren't really engineered for that direction of travel and depending on the steering of the toad, you can do some damage to the front end.
2016 Thor Hurricane 35C with pups Piper and Annabelle
2013 Ford Fiesta toad
FMCA - F457085
Blog - https://traversity.us