You're right - there's no way to save these out currently except by saving the whole mrml scene file. We've been talking about adding a way to save subsets of the scene, like just the parameters for the segmenter or something, and I think this idea of saving the slice nodes is another good use case for such a tool.

For what it's worth, I've written scripts to handle this functionality for myself, but it's probably not a good general approach. If I integrate it more with slicer I'll let you know and/or run it by slicer-devel to see what would be more generally useful.

I've scripted a fair number of slice-level operations for our current usage, and as they settle down it might be worth looking at packaging them up as a module. So far I've avoiding learning (nearly) anything about your GUI architecture though.

Closing. Slicer3 is not being developed anymore.