Sorry but aren't there WAY more easy solutions to this ?
I mean just put an extra gear on the motorplate like so:
(Blatant copy from Jimmy's Euro's report)
Now you can change the rotation of the motor without changing a whole transmission. Just put on the extra idler and change the ESC setting to reverse rotation. That is less than 5 minutes work even when youre trackside. That way you can change it very fast when it starts raining you before your heat / main.
Dont worry about the drag of the extra idler gear, current motors and batteries have power to spare. And you'd be able to run LESS weight in the front.