I spent a good chunk of time looking and I couldn't find anywhere that would allow me to modify the username displayed next to each post.
Currently, my members are able to customize their username using hex codes. While I personally find this functionality trivial and scarcely worth the time I've already invested trying to find the hook, my members as a group seem to enjoy the functionality, and it'd be nice to provide it to them if a proper event is available.
As an example, maybe ol' DynamoGeek is feeling Christmasy around December time.