#if ENABLED(PIDTEMP)
//#define PID_DEBUG // Sends debug data to the serial port.
//#define PID_OPENLOOP 1 // Puts PID in open loop. M104/M140 sets the output power from 0 to PID_MAX
You could define PID_OPENLOOP and send M140 S55 to heat the bed to 55 PWM ( 0 to 255 is possible )
I never tried it, but it should work without thermistor feedback then.
-Olaf