Open Loop BLDC motor control, is it possible?

There's nothing at all wrong with open loop control. You don't need a pwm, just send a continuous on. The outer loop is typically unrelated to the commutation logic.

You do, however, need to correctly commutate. Using the hall sensors to do this is generally easier than sensorless control using backemf measurements. If you can afford it, using a BLDC controller IC to deal with the commutation is a lot easier than writing the code to read the sensors (or back emf) and switch the mosfets on and off. The ICs can be better optimised than naive firmware, too, so you may get small efficiency advantages and smoother movement.

The disadvantage of open loop control is that it's open loop control. You never really know what speed exactly the motor is running at. The same motor can operate at different speeds under different external conditions for the same control input. These external conditions can be incredibly difficult to control, and next to impossible to keep unchanging. If your application doesn't really care about the actual speed, then you don't need closed loop control. If it does, you will need to close the loop. At the very least, you'll need to measure the speed so that you atleast know what speed it's at, even if you don't implement a full PID loop with all the bells and whistles.

If open loop control is used with a BLDC motor, it will not react to torque disturbances. If the motor can't react then you could stall the motor, which means constant torque applications would be best for open loop BLDC control.

Another thing is the coil power must always change or the motor will not turn, so you need more than just leaving the coils on for open loop control. If you leave two sides of the coil on no current will flow across the inductor coil.