Yes, finally after months of twiddling my thumbs waiting for approval to land the zipwriter component has gone into trunk, and is enabled for all applications (some apps might need to add the interface to their packages list). If you want to use check out the interface, it’s fairly well documented I think. That magic contract ID you need is “@mozilla.org/zipwriter;1″.
It’s been quite a long process both writing the code, getting reviewed and getting agreement for it to appear in Firefox so I hope all you extension authors and application developers are going to make good use out of it in your projects. Let me know what you come up with.