This is exactly what I’m after also. However, when I turn on the HTTP authentication in the Admin page I get a bunch of warnings at the top of the screen (see below), and also am unable to log off.
Warning: Missing argument 2 for bb_login() in …/bb-includes/pluggable.php on line 125
and multiple of:
Warning: Cannot modify header information – headers already sent by (output started at …/bb-includes/pluggable.php:125) in …/bb-includes/pluggable.php on line 37