My https://github.com/kabir/wildfly/compare/jca-fixes branch contains some preliminary work on this. In short it adds the placeholder override model when adding the datasource, and removes it when the datasource is removed. When the datasource is enabled, the listener then populates the placeholder override with the children. When the DS is disabled, it then removed the children from the placeholder override.
I've added the placeholder override when deploying xml as well but am not 100% sure where that should be removed.