The only way I know of getting a portrait to work while disguised, is to have its name (- the H/L/M/S/T) in the description section with no other text..
E.G Description: "_knight_" would work for adding the "_knight_H/L/M/S/T" portrait, but adding anything else before or afterwards stops it working.
If there is a command to set the portrait seperately, please can it be added to the in game '@disguise help' command, as currently that does not mention how to set portraits at all.