pkzip does reserve the posibility for an arbitrary length extra field connected to each file. According to the spec (http://www.pkware.com/documents/casestudies/APPNOTE.TXT) this is for "additional information...for special needs or for specific platforms". All compatible zip tools are required to ignore all information in this field that it doesn't understand so you can basically write whatever you want there (although the spec does offer a recommended format for writing to this field). So if you write a special ACL preserving zip implementation, you can still unpack the file with any other zip implementation that knows nothing of your special version.