Is there a method to share a briefcase (that's the case in point, but I suppose it would apply to calendar, todo, et al.) view-only to all members (ie. non-public / valid login required) of the domain?

The only method I see so far is entering each domain member to the share member-by-member.

The options for "external" and "public" share, if I am understanding correctly, both seem to allow public, passwordless, access to anyone who knows the correct url. (On that note, does zimbra build in a .htaccess to make sure the "public" shares do not get crawled??!).