You wouldn't necessarily use mechanical doping to win the general classification, or even a particular stage.
More likely, you'd use it on select stages for very specific reasons... for example, a rider could use it to avoid the time cut on an ITT stage (effectively getting extra rest vs their competitors). Similarly, a pure sprinter could use it to stay in contention on a punchy "sprint" stage (like a stage that MvdP might be a favorite instead of a pure sprinter).
Edit - I don't think anybody is doing this at the top levels of pro cycling. Maybe in regional racing (masters, etc).
If I were responsible for a mechanical doping program, then I'd install the motors for the leadout and mountain domestique riders and leave the team leader clean. Who cares if they pay the weight penalty after peeling off if it means that they can provide extra support for those critical minutes?