when i do my rig shots, the car is literally rolled at no more than 1-2 mph with a 3-4 second shutter.. unless you had training wheels on the bike i dont think you could use the same method with it, lol.
as far as a rig.. since the bike is so much smaller, specially if you had a nice wide angle lens, you could probably find a way to hand the cam off the side and snap a picture at a very high speed to minimize the vibration.. i would bet that at 50-60 mph with a shutter speed of 1/60 you can get really nice motion blur without having too much vibration in the image..
another way i could think of ghetto-rigging a self-motion-shot on a bike would be to do exactly what i just said to get an image of the blurred background and the rider, and then snap another still image of the bike not rolling to get some detail, and overlay the images to bring back some of the detail in the bike ontop of the blurred background.. but seems like its too much effort for something like that