Is there any interface to add a custom header to a nrrd from Slicer, i.e: quality:=incomplete, or populate a predefined existing header to a Segmentation image (seg.nrrd)?
Each segment has a state (not started, in progress, completed, flagged) that you can see and modify using the GUI and also saved in the .seg.nrrd file.
If this is not sufficient then you can add arbitrary number of custom properties (tag:value pairs) to any segment using Python scripting (using vtkSegment::SetTag method). These custom tags are also stored in the .seg.nrrd file.
You can read all these states and other properties in .seg.nrrd files using slicerio Python package (in any Python environment, not just in Slicer).