If you want any advanced formatting options for the AddAndEditNew modal window you have to:
1. Create a new modal window
2. Customize the window
3. Edit the code to have that modal window handle the AddAndEditNew event.

I don't think it's too much to ask to be able to simply edit the default window a little more. I know you can move fields around and choose whether or not they are displayed, but it would be great if you could edit it like any other modal window. I know how to replace the default modal window, but maybe someone new to LS would not.