The model I have at the moment is still fragile and prone to change until I can get something concrete together to use the model and see what breaks, and where, under stress.

Unit Testing has helped in some way, but I'm not entirely happy as yet. So I've not thought about Javascript. My stance is that you put in the document HEAD the Javascript file, and use the DOM to manipulate the document based on IDs, from the client but...

This is another topic I think, as we now have rich clients using AJAX, JpSpan et al How are these effected by a recursive structure? I don't know myself at the moment.

If parent asked its children for <head> info, the child wouldn't even need to know about its parent, in which case the parent my as well be root.
I am finding it difficult to have one composite controller know about another composite controller as I said earlier in this thread, so this is basically what your asking as well. Doesn't really matter if it's the composite with the HEAD template fragment or not, as in a (composite) recursion all composites need to be treated the same.