This is client side only and post was made before about holding down numpad 0 when numlock is off. I couldn't get this to work but here is a new way.
Type /console pitchlimit 9000 this will allow you to fly in any direction, even upside down. To reset to normal type /console pitchlimit 90
Enjoy
EDIT: Posted before I just didn't see it. Sorry for taking your time.