WebLogic and RAC

in fact the GridLink DataSource
http://docs.oracle.com/cd/E21764_01/apirefs.1111/e13951/mbeans/JDBCOracleDataSourceRuntimeMBean.html

contains an array of instances, one per Oracle RAC instance
http://docs.oracle.com/cd/E17904_01/apirefs.1111/e13951/mbeans/JDBCOracleDataSourceInstanceRuntimeMBean.html

plus a ONSClientRuntimeMBean to handle communication with the ONS (oracle notification service) .

One could monitor in WebLogic the status of each individual Oracle RAC Instance...