First of all - don't create JIRAs before we have come to a conclusion on the possible feature.
Second - you havn't posted your configuration of the datasource. The test-connection-in-pool will use the default credentials specified, so why isn't that working ?
However, I do see a possible use-case for an overloaded test-connection-in-pool method, but the core signature of method is different than you describe. Datasources are just one usage of JCA.