Looking for confirmation of a BLDC project
yes and No. 

Yes you can flatten the torque curve but not the power curve.

Since RPM at no load is voltage control and Torque is controlled by V/DCR at 0 RPM and then reduces with rising RPM due to back EMF. 

Thus lowering the voltage to reduce RPM increases available torque and by using full bridge low impedance PWM control the average DC voltage can be controlled and maintain a constant driving impedance. 

In the low speeds higher motor currents when more torque but lower voltage increases conduction losses significantly .

Perhaps you can  improve cooling by some means at low RPM such as a water cooled engine.




Do you really need constant power from 100 to 3000 RPM?

2 sets of 4 pulleys is the most common method with a fan belt.

https://en.wikipedia.org/wiki/Variable-frequency_drive