Mean time to failure (MTTF) of coreless DC motor?
Would it be reasonable to expect a coreless DC motor to have an MTTF
  of 500-1000 hours if the applied voltage was half the spec voltage?


lifespan is mainly determined by brush wear, which increases at higher rpm and higher current. Intermittent running is worse due to startup surge currents, but a lower duty increases the life of the motor by reducing its run time. Therefore it is difficult to give a MTTF because it depends on how the motor is operated. However you can get some idea from the intended application. 

The most popular uses for small coreless motors are cell-phone vibrators and small r/c quadcopters ('drones'). Drone motors are designed to produce the most possible power at high rpm, so they have a short life. Vibrator motors work at lower rpm and current, but are trending towards higher power to get stronger vibration. 

Typical no-load rpm is 40,000-80,000rpm, with smaller diameter motors generally running faster. At half voltage a DC motor will run at half speed, so lifespan should be dramatically improved if the load is light. However even with a light load a high power motor will draw relatively high current, so a drone motor is likely to have a shorter lifespan than a vibrator motor. A larger diameter motor may also have longer lifespan because the brushes and commutator can be larger and more robust, and a longer motor is usually more efficient (due to more copper in the 'active' part of the basket) so it draws less current for the same power output.

I am running 24V 7x16mm coreless motors on 7.4V, doing ~9000rpm and drawing 10-20mA. I pulled one apart after about 50 hours operation and the brush wear was almost imperceptible. Unfortunately all the sources for this particular motor seem to have dried up, as modern devices typically run on 3.7V or lower. However a few manufacturers (eg. Namiki, Citizen) still make higher voltage low speed coreless motors for industrial applications. These are expensive, but should last much longer than the 'generic' coreless motors used in toys etc.