How can a BLDC motor respond to a specific dynamic load demand without changing the supply voltage?
If I use a PWM controller at 50% (but the supply voltage is the same), is the torque-speed curve really going to have a characteristic curve that responds as if the supply voltage was half?

More or less sorta kinda yes.  It depends on some details of how the driver is implemented, but if the drive truly has a 50% duty cycle, if it does not have much dead time in the gate drive, and if the voltage is brought low in the low parts of the cycle -- yes.
Even when the PWM to the motor is looser the motor drive will still generally be monotonically increasing for increasing duty cycle.

If not, how are these BLDC motors controller for dynamic applications where a specific torque and speed are needed at different times?

They have RPM governors.  You need to know the position of the armature with respect to the stator to make a BLDC work, which means you have all the information you need to determine the shaft speed.  If you want a constant speed (as with a hobby ESC in &quot;helicopter mode&quot;) then you have a control loop that monitors the BLDC zero crossings, times them, and uses that to drive the duty cycle.