I haven't seen it happen, but another example would be that people can learn numbers without becoming functionally literate. Numbers are a much more restricted set of symbols.
The dumb thing is that functional literacy actually circles back to the same principle of shape processing that idiograms rely on - thinking of that spam where they scramble all but the first and last letter of a word and you can still read it. The effect is even more pronounced in handwriting.
P.S. I've also been contemplating how this relates to Peircean Semiotics. Going back to the relationship between Cantonese and Mandarin, one theory would be that the visual sign is primary to the verbal sign.