In the demos, I used this approach to render segmentation masks on the render windows.

previewImage->InitializeByLabeledImage(outputImage);
previewImage->SetGeometry(this->GetWorkingPlaneGeometry()->Clone())

While this won't work when confirm button is clicked (since the geometry is not of an image).
The proposed solution was to use:

mitk::SegTool2D::WriteSliceToVolume(
       previewImage, this->GetWorkingPlaneGeometry(), outputImage.GetPointer(), timeStep, true);

This 'works' but produces a plain full mask for the slice.