I think there's something to be said for a solution like this, though I'd just require them to enter more than 6 characters for their username in the first place.

I would be wary of trying to generate usernames from the user's real names simply because it seems problematic to try to use non-unique data to generate something unique. Since their real names aren't required to be unique (and can't be required to be), they're not a good starting place for usernames (which must be unique).