Taxonomies limited by parent node taxonomy

I have trouble expressing my problem in one single sentence, but I think what I would like is to select only taxonomy entries which are available only in the parent node, referenced via a node reference.

I am currently trying to patch together a "Living Stories" (Youtube video, summary of the features) feature. One of the requirements are themes within a story.

For now I have modelled this as several nodes:

Living Story

The living story, this I will add a template, panel or similar to style it and add appropriate views

Living Story Update

Basically a story which have a nodereference field for selecting the Living Story to update.

Living Story Source

Nodes with data/donwloadables/graphics, and sources for those. As per requirements.

Now, the themes are like sub stories to the main Living Story. I originally were thinking of creating these by using Taxonomy and Hierarchical Select. When creating a Living Story the user would have to copy the title into a new root node in the selected Taxonomy.

Themes would be added as sub-terms of that root node.

But, it quickly dawned upon me that this would not work if there is a large number of living stories, it is irritating to look through lists longer than a few tenfolds.

My next idea is to create a node type "Living Story Theme" and let those have a nodereference field for selecting which "Living Story" they are a part of. Then in the "Living Story Update" I would have a nodereference field for selecting which "Living Story Theme" the update is part of (in addition to the Living Story).

The thing is that even if I create a view which just lets you select a subset of the "Living Story Themes" based on the "Living Story" selected, I can't see how I will pass that argument into the view upon node creation, since no theme has been selected yet.

One solution would be to add a link which uses one of the modules for passing info through the url to the node creation form, filling in the value of the "Living Story" at once; but still I don't have access to that in my view.

Next step would be to add the "Add update" link under each theme, or allow the user to select which theme to update before getting to the node create form. And if I use node templates I can't see how to pack them in from the Features module (now there is a new module idea for enterprising coders).

Any clues on where to go now, would be welcome.