i worked at Calfee, and we would repair broken carbon frames. It would be quite easy to cut open a fat down tube, place a big battery in there, and then repair it, and paint over. perhaps the sound of the motor wouldn't be heard if it is kept at a low enough speed? so it isn't as beneficial as it could be, but if it's giving you a 50-100 watt boost, that's very substantial... you're opponent who is approximately evenly matched will be at his lactate threshold and you will be below it keeping yourself fresh and ready to attack.
i'd like to see beppu or arashiro start contending the TdF only to be caught with the latest in biketronics from shimano hidden inside their frames.
i just realised something! with how powerful the draft is when you are enveloped in the peloton, you could even recharge the batteries without doing any real work! if you stuck to peoples wheels, this battery powered bike could weigh 10lbs extra and you wouldn't really suffer a penalty, because you'd activate it on the climbs, and recharge it the rest of the time. perhaps an adjustment that lets you fine tune how fast it is recharging on the fly, so you don't induce too much drag when you can't afford it.