This will do it, as long as your choices are one of these types "All Visitors", "Members Only", or "Members Only (Hidden)". The other choices all require more detailed modifications (e.g., passwords) to each forum. This only changes existing forums, new forums would have to be set individually (or you could repeat the setting here at any point). If you want a setting which will affect future forums added to a category, let me know. I can code it for you in an hour or so.
In "post.asp", look for the following bit of code (appx lines 1011-1019):
Below that, insert the following:
In "post_info.asp", look for the following bit of code (appx lines 73-79):
Below that, insert the following: