I understand now - the answer is no - from page 56 of http://pubs.vmware.com/vsphere-55/topic/com.vmware.ICbase/PDF/vsphere-esxi-vcenter-server-55-host-management-guide.pdf
vCenter Server does not support Linked Mode groups that contain different 5.x versions of vCenter Server or different versions of vCenter Single Sign-On. After all 5.x vCenter Server and vCenter Single Sign-On instances in a Linked Mode group are upgraded to the same 5.x version, you can rejoin them.