All that works for me is putting T_White in the individual mask inputs.
On a related mask note, how would I go about layering two versions of an effect, say I want one mask on the monitor effects lines and a different mask on the distortion? I could duplicate/edit the base mat but that’d be a hassle every time I update. If there’s a more sensible way, I’m all ears 
EDIT: Got as far as establishing I’d need to edit both the PP mat itself andf MF_SetBlending to add a second mask, and decided to give up before I get in a right old pickle Probably easier to just use another instance of the mat in a standard PostProcess volume.