ok, this is untested, but I did a version of this on my 6.5 board. Note that this is using code from an old 6.3.0.0 upgrade zip, so it might look different in 6.3.1.2
in public_edit_profile.pl
find:
[/code]replace with:
[code]
that'll not only not allow anyone from choosing hidden, but it will also set it so anyone who has hidden already chosed will become unhidden once they update their profile.
Remember, this is untested, so backup the file before you modify it.