It'd be really nice to have that option, as I have many times wasted my time downloading a bunch of files only to discover that they needed a password at the end. If not a filter, at least showing which ones are protected with a different color. Binsearch already provides this info in their search results (they put [requires password] in red right after the collection name). In fact, alt.binz does not show it, so it's already being accounted for.
Also, as a new member, cheers to all! And thank you to everyone who contributed to make alt.binz such a great software.