Peter,
I am not surprised you are having difficulty finding anyone with much experience in hydronic heating. It is relatively new industry in Australia. Dare I say Tasmania is the best market in Australia, although a little small, but even here it is difficult to find good experienced operators. It is probably no coincidence that David and myself are the only ones to respond to your request so far. I am sure David will know of many home made and semi professional systems running off wood fired boilers, slow combustion heater or even wood fuel stoves. i know of quite a few - they are not that uncommon down here.
I had a Solar Water Wise designed and supplied solar hot water system installed last year. The plumber who installed the system specialises in hydronic heating and solar solutions. His name is is Zlatko Paksec and his simple web site is at
www.hydrosol.com.au. He was excellent. When talking to him about his interest and specialisation, he mentioned that he had virtually no competition in Hobart.
So, whilst this does not help you directly I guess the message is that the more careful and patient searching you can do in this emerging area the better your chances of finding the right supplier and installer.
Cheers
Steve.