Second the idea for “forcing” folks to use their calls as their login user name, and changing the UI to stress that. If someone doesn’t yet have their ticket, they wouldn’t necessarily be required to use a callsign, however, and I don’t see any easy way to force folks from using something else if they want to, in which case the UI text changes could be a little confusing for those who didn’t use their calls.
Otherwise, AFAICT you’re pretty much stuck having folks use a custom field, but not sure how you could get that to display alongside their posts.
Good luck! 73 de N2UN