Seeing how little room the currently used font takes and how much room is available, I think the markings could be fit in addition to the current markings in the center of the keycaps. Obviously, 3-letter abbreviations would fit. Small pictograms could work nicely.
If the users agree that the current alphabet markings are superfluous, we would have a nice clean spaces for micro-art.
If You wish so, this could be the start of the twiddler pictogram art contest with the winner pictograms appearing on new twiddlers