Its a service within AS with a name of ConnectorServices.IRONJACAMAR_MDR, so you need to add a dependency on it from your code and get it injected - ...addDependency(ConnectorServices.IRONJACAMAR_MDR, AS7MetadataRepository.class, service.getMdrInjector()).
First you need to hook up the MDR to the DMR model under subsystem/jca/mdr, and make operations available to query it. Then the domain model infrastructure will take care of the rest, and you can have your standalone client access it that way.
I would start here - https://community.jboss.org/wiki/DetypedDescriptionOfTheAS7ManagementModel