People who say it cannot be done should not interrupt those who are doing it.
Far from being low in embodied energy it seems our slab is overengineered and most likely uses more concrete and steel than we bargained for.
Our slab engineers seem to have gone overboard and added a lot more in the way of steel-reinforced concrete footings than was absolutely necessary, but not being a structural engineer myself or knowing a thing about soil profiles and the like, I am hardly one to make such a judgement. However, our builders had certainly never seen anything like it in a building of this type, and this is what council approved so this is what gets built.
We had done some research on more environmentally friendly footings and foundations and Zeobond E-Crete seemed like a promising alternative to portland cement but unfortunately it is not yet available in our area. I gather they still have only a research facility and are not producing quanities for domestic residential dwellings.
I've also seen building posts that have been secured in the ground with a web of steel rods that criss-cross around the post to provide stability without any concrete footings. I'm not sure what the difference in embodied energy would be in this case, but it's worth investigating.
We didn't really feel that either our structural or hydraulic engineers were all that responsive to our requests for information or alternatives, but in their defence they were contracted through our draftsperson, so communication was not as direct as it could have been.
I think if we had to do this again, we'd spend more time trying to find professionals who had knowledge of and proven experience in utilising newer, less harmful options, and were interested in listening and tailoring a solution to our needs rather than just providing a stock standard service.