[polivier]

> would just be a very heavy bike for the rest of the stage

Bikes in the Tour de France have a minimum weight of 6.8kg imposed by the UCI. So if you manage to build a normal bike that weights 5kg, you still have 1.8kg of weight available to try to add some more hidden power "without adding more weight to the bike" (small battery+engine, small compressed air tank, whatever).