The Oracle BI Publisher Enterprise Cluster Deployment doc which I just found through Metalink highlighted a couple of points:
- Report repository should be shared
- The scheduler should be configured for a cluster
Through Admin>System Maintenance>Report Repository I changed the path from the default, /xmlp/XMLP to a NFS mount data/shared/xmlp and restarted the xmlpserver application in OAS. On coming back up Publisher complained because all its config files (in xmlp/Admin), had disappeared.