I have a basic brivis system where you cant control the output power of the compressor. So on a system like this it is use less designing a system to work with only one room.
Secondly the more duct work you have in the roof the less efficient the system the system is. Just because you insulated it doesn't mean it will not heat up.
I guess in a big house (very big house) this will be worth looking into, or even double storey houses. It should be possible to do with most system with current technology I have already thought about this previously. All you need to do is wire two zone motors on each relay. One for the air outlet one for the intake. But it still a little more complex than this.
Several bloggers have said that the Actron ESP can turn its air flow down to deal with just one room. This is also what they claim on their website.
I take your points and when we get closer to actually choosing our ducted air conditioning for our new house I will raise my ideas with the technicians and it will be interesting to see what they say.