Currently vtm (multiblock dataset format) is written for poly data master representations, but a 4D NRRD is written for labelmaps. The problem with 4D nrrd is that it may have huge memory usage especially in case of many segments, because the union of the extents are used.
It would be better in this sense to use vtm file (which is an XML pointing to files in a directory with the same name) for labelmaps too, because the segments can be saved one by one. Other advantages:

• The resulting NRRD files can be loaded as labelmaps individually if needed
• vtm can be the file extension for segmentations in Slicer, and we could get rid of the extension problems too (0000758)

Implementation: vtkXMLMultiBlockDataWriter::WriteComposite writes the individual blocks into files, but for images it probably uses vti. It will need to be subclassed to use the NRRD writer. It can be done by also subclassing vtkXMLCompositeDataWriter and reimplement CreateWriters so that it creates NRRD writer instead of vtkXMLImageDataWriter