The term is 'regeneration', not 'recuperation', and the returns are very small. It was a big deal, and selling point even, when e-assist was a novel idea, but how many regenerative systems do you see out there now? It's not because it's difficult to do. All the raw material is right there but in practice you return so little energy back to the battery that it just isn't worth it for most. If it isn't worth it for the primary drivetrain, it absolutely would not be worth it to source an additional motor/generator to drag (literally) around just for the 'free' energy from braking and downhills.