In order to get exclusive or SCSI access for volumes in MacOS.app, the Workspace Manager must be able to unmount them in Mac OS X Server. This is not possible if there are files open on them in Mac OS X Server or if someone is logged into Apple File Services (AFP server).
There is one potential issue with the AFP server. If no one is logged into the server and MacOS.app is launched with volumes set for exclusive access, those volumes will be unmounted from MacOS.app. The result is that AFP users cannot mount these volumes until MacOS.app is shutdown. Again, if someone is already logged in, this will not happen.