As I think you understand, with dye sublimation you cannot produce colors lighter than the surface on which you're printing. If you have an extremely dark keycap, then you can't really print any darker to have any sort of defining contrast between the keycaps and the lettering. This would make lettering almost invisible, and most people who want lettering want their lettering to be visible, so we probably wouldn't be able to reach MOQ even with an option for lettering.