There is no question that this CAN be done. A previous poster described model airplane motors and lithium polymer batteries. I fly these planes, and 100W for 20 minutes is a simple thing. 500+ watts for 3 or 4 minutes is also simple.
The biggest issue would be the noise. Electric motors driving gears would not be quiet at 100W outputs. But cobbled sections with screaming crowds would easily drown it out, and once you pull away...
The weights are miniscule. Hi capacity batteries and powerful motors are available in such variety that you could find a motor for any size tube. The batteries are generally thin, flat cells built up into packs like small bricks, but cylindrical cells are also available and made in large volume for aerospace, military, computer and other applications.
The other side is the custom bike. Modern composite materials can easily be used to shave a kilo or two off a bike frame.
This is possible. Whether anyone in the pro peloton is using it I can't answer, but I theorized it with friends 3 years ago.
If you like I could post photos and videos of some of the tech available, but just perusing a site like this
http://hobbyking.com/hobbyking/stor...ll+Inrunner+Motors&idCategory=64&ParentCat=59
or
http://hobbyking.com/hobbyking/stor...me=Turnigy+Lipoly&idCategory=317&ParentCat=85
should conivince you it can be done.
In the electrical world, Watts = Volts * Amps. So to get 100Watts with a "4S" pack (14.8V) you have to draw 6.75A from the battery. This is very low power for these systems, and a 412g, 5000mAh battery pack could support that output for about 45 minutes. Since brushless motors are about 80-85% efficient, and there would be some efficiency loss in the gearing, expect 60-75 Watts going to the wheels during that time.