Im sure that the cops wont turn to cops, as its an organized event for car enthusiasts of japanese cars, to meet, cruise & socialize. If ppl attending behave themselves and not pull any stunts, then the cops shouldnt have any reason to defect/give a hard time to anyone. Lets just say that this event disaprooves of any illegal activities eg. drags, burnouts on public roads. To the local police surrounding the meet point- you are more than welcome to monitor the cruise.. as you certainly will be doing anyway.
I think you're being a little naive if you think all drivers who attend a large cruise event such as this will be well behaved. It WILL attract a certain number of dipshits who turn up and rip burnouts, etc. You may not condone the behaviour but it doesn't stop it from happening.
Having said that, good on ya for putting the effort into trying to organise cruises for Melbourne car enthusiasts! Hope the night runs smoothly for you.