It would be great, but the current CLI module logic doesn't support it, so it's more a feature request than a tweak.
I think it would have to be another flag on an output data type to auto create a node before calling the CLI, but I'm not sure that would work with the current infrastructure. In this case the hierarchy node has to be passed via a scene file to the model maker so that all the models can be read back in (as that was the only way to support multiple output models).

This has been fixed in r23976

ENH: Jim Miller's fix for enabling Hierarchy sub-scenes to be passed to CLI
See http://viewvc.slicer.org/viewvc.cgi/Slicer4?view=revision&revision=23976

This issue is concerning the user interaction with the model maker CLI: they have to manually create a new model hierarchy node or it won't run. The revision that @jcfr referenced shows a top level check of if hierarchy node exists then do the passing, and that would be a place to investigate if we can auto-create the hierarchy node if it is null.

Quick look at the current state: there isn't a good way to tell that we're missing a model hierarchy node when it's required at the cli logic level.

The model maker apply button wasn't active when I did the following:
load a label map called seg
enter model maker
When I created a new model hierarchy, the apply button became active, but the GUI reset the input volume node back to none and I had to reset it to seg.
To investigate: required arguments vs active Apply button.

There is also still an old interface being referenced in the error message (references to Slicer 3 and scene import):

ERROR: no model scene defined! Using CDHEF_vtkMRMLLabelMapVolumeNodeB.mrml
WARNING: If you started Model Maker from the Slicer3 GUI, the models will NOT be loaded automatically.
You must use File->Import Scene CDHEF_vtkMRMLLabelMapVolumeNodeB.mrml to see your models (don't use Load or it will close your current scene).

Model scene file doesn't exist yet: CDHEF_vtkMRMLLabelMapVolumeNodeB.mrml
Error: no model hierarchy node at ID "", creating one