I spent a little time this morning attempted to call some people in Australia who may have more of an insight with regard to use of magnetometers in this particular area (in both senses of the word 'area'!). Unfortunately half of the country appeared to be out or in meetings but I hope to get a response in the next day or so. If not I'll try again next week.
@Sunfish: Interesting to hear of your experience, I suspect from your description that may not have been a magnetometer per se(?). From what I understand there have been some advances in the field over the past 15 or more years such that a caesium unit could be somewhat more useful for this task than what you were previously using. Certainly I think it's worth persevering with the enquiries at this stage.
Totally agree with the testing aspect prior to taking it to site, the area does sound rather inhospitable. As it happens I have a very similar site available (~200 acres of what might be described as 'jungle' on a hillside), so could trial something, but I'm a wee distance from you and don't have the necessary sensors available. As a lightly amusing but related aside I do have access to a [twin] wasp engine and also own a bulldozer, but I won't be carrying out any testing to that degree of realism
FP.