A datasource's <valid-connection-checker> element lets you specify the class name. It does not let you specify the module name, so you can only use IJ-provided ones. It should let you specify a module name too, so you can use custom connection checkers.
Jesper Pedersen <jpederse> made a comment on jira WFLY-2492 Extensions should be deployed as a module, and linked with the JDBC driver using the module.xml file
WFLY-2492 needs to be resolved before this will be considered. EAP 6.4 is out of scope, as the issue is unresolved and would require a subsystem update. Workaround exists for the EAP 6.x series.
Lin Gao <lgao> updated the status of jira WFLY-2492 to Coding In Progress