New in ZCS 5.0.1: company B users can visit
https://mail.companya.com/home/bugle.../?fmt=freebusy for a human-readably HTML view.
/?fmt=ifb is a publish URL for Outlook-compatible free/busy data.
It is possible to integrate this into Outlook, but not user-friendly -- Google for "outlook free busy without exchange." You have to set an IFB URL attribute for every company A user in every company B user's addressbook. It might be possible to manage this centrally.