Hi miki,
Excellent description of your project and problem.
motorPWMLevel[]) may do what you need. Wtih PID on RobotC tries to maintain speed. So if resistance is encountered pwm level goes up. This could be used as a torque indicator. Below is a short program that displays the pwm (pulse wave modulaton) level on the nxt screen.

while(true)
{
motor[motorA] = 10;
nxtDisplayBigStringAt(20,30,"%d", motorPWMLevel[motorA]);
wait1Msec(50);
eraseDisplay();
}

Mount a wheel directly on motorA. Apply some drag to the wheel and watch the number go up. Remember to keep pid turned on.

Merry Christmas,
Dave