For one thing, a 747 is only 250 feet long. So a 300-foot blade on top would trail significantly.

For another, the shuttle piggyback worked partly because of the shuttle’s aerodynamic profile. It’s designed to go straight forward into the wind. This means the 747 still handled well. A windmill blade, though, would present a very different cross section to the oncoming air, and seriously screw up the host’s aerodynamics.

So you could mask it and put fairings on it, and by the time you’ve done that 500 times you’ll wish you’d just built a special-purpose airplane to begin with.