Hi,
From perth here and though i could give you some advice,
the standard procedure for boats getting under bridges in perth is using two spinnaker poles, attached at the toerail on the same line as the mast tabernacle (the hinge base)
the two spin poles go forward and meet at the forestay, so at this point you have an A frame, without the cross-bar
Attach the forestay to the two spinnaker poles, and then attach a block and tackle to the underside of the poles and the forestay deck fitting.
you can now ease off the mainsheet block, allowing the spinnaker poles to pivot back, lowering your whole rig.
this is the most stable way to lower your rig, although some wiggle will occur from the tabernacle, just put a bloke there and he can reduce it as much as possible
also check out this site, basically gives you images for what i've just said:
http://www.skymaze.com.au/Mast%20Lowering%20Systems.htmlhere's a picture of a tabernacle which would probable cost about $500-$800 to make/install