Hi , I did it this morning and ... it looks Ok . I verify that at home on the game.
First , I set the backgroung of image in white (not grey like yours). then , I create a specular map of the image . You copy/paste your original image , you change the white part in dark red , then you put a new layer on it , and turn opacity between 50 % and 80% ...
So , I have 2 files , T_Body_D and T_Body_S.
I put the 2 files in UE , Saved all , edit each one and tick on level detail / Never stream and save.
and ... result , I have 2 files for body_D Uasset and Uexp and 2 files for Body_S ... Uasset and uexp .
I put them in UnrealPak when I come at home to verify