No, for disabled icons they will just start with DISBNT instead of BTN, and you have to put disabled ones in
ReplaceableTextures\CommandButtonsDisabled\DISBTNYourIcon.blp/tga
I had no success with this - I've concluded its the program I am using, but I never did download the freebie.
I saw a Nintendo TD the other day loaded with custom icons... so its gotta work!!!!