Any options for over-riding the username restrictions?

No argument here, I absolutely do not want to fork the code base. That being said, the closest I can get to my desired level of integration would be to:

  1. put the user’s name into the Discourse Full Name
  2. check “enable names” in the settings to show the user’s full name
  3. use CSS to hide the display of the Discourse username

Unfortunately, when creating an “@” reply, the Discourse username would be used.