The annotation hierarchy nodes should have associated display nodes. I think that making them vtkMRMLDisplayNodes with the basic set of attributes (visible, color, selected color, material properties, etc) and then having a basic property editor widget (lots of the elements are repeated for the text, point, line display node widgets) that sets the values on that node would be a good start. Then when the hierarchies are collapsed, the nodes under the hierarchy would be displayed using the hieararchy node's display node properties.
To actually change display properties on groups of fiducials (instead of just over riding them temporarily) will hopefully be a superset of the over ride methods, though get X display node is defined on the logic to return text, point, line display nodes for a given annotation node.

There is Libs/qMRMLWidgets/qMRMLDisplayNodeWidget

So, short answer: needs some redesign to support the full request, and agreed that it's important.

fyi: can set colour for child fiducials if you open up the property panel on the hierarchy node and keep the Apply to List checkbox checked.

For RSNA 2012 will expand the "apply to list" from current color to include size and glyph type, any others to request?
From 2226: label visib, glyph and text scale separately.

Pushed back to post-RSNA as not to be used for tutorials, will implement before if have time.

had some time while recompiling the mrml core over and over today. svn 20860