Hi Greg,
What you are trying to accomplish is more a low level driver test. In the function MACNET_initialize() from MQX driver. Perhaps you can activate this option in the flags for the enet_ptr->PARAM_PTR->OPTIONS variable. Try changing this in the structure ENET_default_params[] (init_enet.c) in the 3rd element.
We don't test the loopback feature so I am not sure if this change will allow you to fully test it.
Hope this helps.
Regards,
Garabo