That's a seriously pain in the butt, weird covenant you've got there Tezz. They actually refer to "Hardie's" products in the paperwork? What happens if you choose a competitor product 
We have something similar but just brick stone '"or the appearance" of such. In the end will just try and bypass. He covenant is over thirty years ago and the area already has a few 'violations' so not too worried yet.
dlvb19 I can understand your issue with concrete and foam, my wife is also concerned with vapors potentially, will have to look into. My main concern is that the more truly sustainable options, like staw bales, I think have questionable longevity issues. If we go down the ICF path, yes there is a lot of embodied energy, etc but it is a property that conceivably have a century or more lifespan without needing further maintenance structurally. I plan to life in this new home for many decades (that's why I am taking such a long time planning to get the core basics right) so lifespan is of importance on top of other practical considerations.
Never an easy, straightforward path.
Tom, I wouldn't be too concerned about alterations. How often will you need to change the external walls (internal walls will be standard frame and plasterboard)? I recently wanted cable Internet added to my current brick veneer home, and until I told him a definite 'NO!' the Telstra contractor wanted to come up through my polished hardwood floors! Eventually I convinced him the internal wall was doable (he winged about drilling through the stud work).