Hi, While i have never tested a belt driven Z axis with a direct brake i did have some very good success with balancing the Z-Axis using a long expander rope. Basically i attached it at the Z carriage, and ran it up and down the entire Z axis a few times using pulleys. With the increased length compared to the Z axis travel, the force is fairly linear, and you can easily calibrate it to be "load neutral".
Cheers!