My concern is based on
Bug 36534 - Syncing a reply to instance of a "noBlob" recurring meeting causes a C++ assert and
Bug 35600 - Deal with change in GetAppointmentResponse for appointment with short meeting notes
I did not get an answer to my thread at
Conflict 5.0.16 server and pre-5.0.16 ZCO?, but the above bugs suggest to me that any ZCO pre-5.0.16 will have problems with a 5.0.16 server.
We are still on 5.0.15 and (more slowly than I'd like) upgrading all clients to 5.0.16.2... which we're going to want anyway in order to support Office 2007 SP2 *or* the pre-SP2 Outlook hotfixes.