I was posed with the question as to how to protect a BB forum with a general user/pass for students at a community college. Since the segment that needed securing was not a standard post, but a custom rewrite, there was no way to use the native post password feature. Using .htaccess was also out as that protects entire directories.
Fortunately, PHP allows you to implement auth headers to handle this. Let me first say that I understand that HTTP Auth Basic is not a secure authentication solution, but it is a privacy gate which is all that was requested to help reduce spam, malicious posts, etc.. With the proper hooks, I was able to target the specific URI segment and it worked like a charm.