On the 331-200 APU (B757/B767-2/300), a "NO FLAME" shutdown would occur if there were no fuel in the tank. A "NO FLAME" shutdown occurs if :
- APU Speed is greater than 7% or more for 14 seconds
- EGT is less than 204°F (400°F)
You are correct in that the "APU Starter motor is capable of accelerating the APU without the aid of combustion up to a speed of approx 20%".
Butafter the alloted time, the "No Flame" shutdown will occur. The APU ECU will remove power from all torque motors and the APU Starter Motor. So to answer your question,...NO, the Starter motor will not continue to run until it craps out.