Others like this have been on the market for years, in particular a flatbed trailer with the Currie Electrodrive unit mounted below the platform and driving one side wheel, this sold by Tony Castles of Brompton Nano fame.
It's essential that the trailer hitch is mounted low on the bike to prevent the one sided drive upsetting the bike balance too much.
As Indalo suspects, illegal since it's effectively a four wheeled e-bike which is not provided for in law.