I had encountered the same problem and found a creative way to overcome it, because it seemed there's no real solution as these characters are omitted in the system.
My solution is quite straightforward. you take the location of the label after it's drawn and add a text in a proper location using the default TEX interpreter with only the permil sign. The location comes out correct in most cases, but can be adjusted if not.
-it doesn't always work with "xy(3)/2" and "xy(4)*1, so the factors (2 and 1) might need to be changed.
-note that Latex interpreter always draws in somekind of grey, therefore I specifically choose 'k' as color text color when Latex is used.
Hope this helps,