You can do anything you want if you write your own program... looking at the link, I don't believe the author provides anything other than a HEX for his projects.

As for other characters... you can have whatever symbols you want, as long as they are made up from the available seven segments. This becomes a problem if you want to display characters like K,Z,M,W,Q,X,V etc (as there are no diagonals)... some like Y can only be displayed in lower-case.