PWM as a voltage regulator for 12V wiper motors and 14.8V LiPo
Yes. For short transients it's likely safe. If you have motors to spare, try the same thing at 14.8V while stalling the motor. Often automotive components are design for 9V-16V, and motors are designed for stall, as you can't have them smoke when something faults. 

The only real limit for these motors is thermal and the gear train tolerance. The bearing wear increase for higher speeds is likely not an issue for your application.