In the Render Settings > Common tab, choose EXR as the output file format.
In the Render Settings > AOVs tab, open the drop-down menu on the right side of one of the AOVs in the list, and choose Select Driver.
This will bring up the defaultArnoldDriver node Attributes.
In the defaultArnoldDriver node Attributes, under Advanced Output, check Merge AOVs.
In the Render Settings > Common tab, set the output file format, and in Metadata, set Frame/Animation ext. select an option that isn’t Single Frame.
* This will make the Frame Range settings available.
Set the Frame Range.
Set the Maya UI to Rendering and choose Render > Render Sequence.
Render a test with White Balance set to Custom with pure white color.
Let the rendering go on at least until the pixel values stabilize (the Denoiser works at least once..).
Test done using custom white balance with pure white
set the white balance custom color to a color sampled from the VFB at a point that you know should be appearing completely neutral (neither warm or cold)
White balance is set using sample from the test rendering
Render again.
A new render done with the sampled white balance color
* Each time you want to test different sample locations you need to sample the pixels from the test rendered with custom pure white.
Maya
The procedure in Maya is identicle, Add V-Ray Physical Camera attributes to the camera and enable them,
Create a test render with pure white White-Balance Color,
Sample the white/grey surface color in the V-Ray VFB (Render Window) to the White-Balance color, and render again.