Is it safe to over-volt brushless out-runner dc motor on low load conditions?
If you want to run the motor at a higher speed, the most serious mechanical problem is the rotor bursting, though you will also have higher windage losses, and there is also the potential for unbalance vibrations. 

Windage load

These rise as the cube of the speed. As you say the motor will have low load, you may be able to accommodate the extra power to overcome this loss  without overheating the motor. The cooling will improve at higher speeds as well.

Rotor bursting

The bursting force on the rotor rises as the square of the speed. This means a 50% increase in speed results in 1.5^2 = 2.25 times the bursting force. 

With an outrunner motor, the rotor consists of permanent magnets, and an iron return path, that is, a composite structure. How much strength margin has the manufacturer built into it? 

Before I used a motor at significantly higher speed than rated, I would spin the motor (either electrically, or passively by using an electric drill or something) to at least 2x the intended maximum speed. This would give a strength margin of 4x.

Unbalance vibrations

A motor does not need to be as accurately balanced at low speed, so it may not be well enough balanced for a higher speed. Run it through and a little above your intended speed to check.