I'm adding serialization & saving support to the new FeatureSpace structure data processing utility: https://github.com/keras-team/keras/pull/17355/files
I was happy that it was this easy to do. I didn't even have to make it a Model or a Layer (though I did make it a layer for a separate reason -- to make sure a subclassed layer could own a FeatureSpace and have it tracked for later saving)