Hi, I've arrived at this forum after conducting exactly the same search for Melbourne ('commercial property guardian', 'vacant property guardian').
The reason I'm searching (and believe that this kind of thins *must* go on) is because a friend of a friend said that when she was in Melbourne about 10 years ago, a couple of her friends were acting as vacant property guardians for a wing of a hospital in Melbourne - she said that each wing of the hospital had a guardian in it because each wing could potentially be squatted/vandalised.
I've got a few landscape architect friends in Melbourne who I've asked to enquire for me - landscape architects are all up on and into sustainable living and utilising empty city spaces, so I thought that might be the right circle to ask in.
Anyway - if I get any further, I'll post what I find out here on this thread - if anyone else has found anything more out, it'd be great if you could share your findings too!