With enough money and time, right ? Wouldn't you have a small transmission that would be behind the engine that would be solely for the reverse, and then have a shorter drive shaft ?
And who knows, maybe it was an option for his trike the original owner didn't choose, and just a matter of taking it back to them ?
Just thinking out loud, the bigger the machine, the more important the reverse. I know the big BMW has reverse, I could be wrong but thought I saw a Goldwing one time with reverse.