If that field is set, don't just allow display name & email checks or what not, but 'fill in' a member's settings. Like set the board's default language, default editor, time offset, etc. to the member account as well.

When you create a custom login method, you can do any of these things that you want. For instance, in the OpenID login method (for IPB3, but I did the same when I created OpenID for IPB2) I set birthday (and in IPB2, gender).

For IPB2, you just have to do the member insert right in the login method, instead of using the centralized one. In IPB3 you can pass more data to the create member method.