A belt or shaft drive would work perfectly fine with an internally geared hub. Remember in a Harley the belt is not part of the transmission system. The belt only transfers power from the transmission to the rear wheel and does not change gears itself. On a bike, the chain is actually part of the transmission - it moves from one chainring to another or from cog-to-cog in the back. A chain is efficient because it does not stretch along it's long axis in normal use, but the joints between the links let it angle a little bit between rings and cogs (look at your chain when it's on the big ring in front and the biggest cassette in the back, for example). This would not work as well with a belt drive. Belts can be designed to not stretch - such as in a Harley motorcycle - but they would not allow much angle when changing gears. Also, there is no removable link in a belt - they are continuous. To get a new belt on a motorcycle you have to remove the pulleys on which the belt rests - a huge job. Cetainly one could design a belt to meet all the needs of a bicycle but it would probably be very chain-like! Why not use the real thing which is light, efficient, and can be removed without taking off your crank and rear cassette?