As can be seen from the below debug output from the jboss-as-5 plugin, the property's type is SimpleMetaType.STRING:

Properties for DeploymentTemplateInfo [NoTxConnectionFactoryTemplate]:
...
name=config-property, mappedName=null, viewUse=NONE, required=false, type=SimpleMetaType, value= <<<null>>>
...

but the value's type is supposed to be MapCompositeMetaType(SimpleMetaType.STRING) - this is what the type is for the "config-property" property in existing Connection Factories or Datasources.