I'm surprised ctor hasn't suggested installing the chain inside out or changing the orientation of the tire direction arrow to obtain reverse
There are several complicated ways to make this happen and one very easy way if we only had someone who could create new firmware editions.
Reversing the dc polarity isnt going to work as the dc is "converted" to AC to drive the motor in a specific direction based on the firmware knowing where the motor position is at all times. Rashid et al can set us straight on this concept, though...
The complicated ways I can imagine all require a separate system to either drive the existing motor backwards when the Alta system is disabled or drive a hubmotor in one of the wheels. (the latter would sort of have additional benefits such as 2wd)!
The other way would be to incorporate a mechanical reversing gear into the geartrain next to your right boot. Might as well add a clutch there while you are at it.