Marc van Elburg has sent along a paper he's written about "grawlixes," which are the random symbols used often in comics to omit profanity, like $#!+! You can read his essay on it here.
He follows many of the categories proposed by Mort Walker's Lexicon of Comicana, though he departs a bit from that as well. His discussion also expands from grawlixes specifically to discuss aspects of the "visual language lexicon" such as motion lines and others. He then expands the discussion beyond what is found in comics, to exploring how these signs could undergo additional rules to make them more systematic.
It's definitely an interesting read. I don't completely agree with his categorization and conclusions in their entirety, but I like the approach of systematically cataloging various graphic signs and testing how they work in various contexts. As I argued before, I think that these are the only ways you can really get at understanding how the system of the visual language used in comics works.
Edit: And curiously timed, here's an article by the linguist Ben Zimmer on the origins of grawlixes!