Is there any1 still here? I just signed up.
Anyway.... I'll add my 2c. hehe.
I used to work at the Motorplex, there's close to zero chance that they will let you guys run in their carpark, or anywhere. Reason being is to open the gate they need their security guys there to open up and hang around while you guys are riding. This costs money. There's a whole bunch of other reasons why they wont let you run there no doubt. If you do want to try, call the motorplex and ask for Peter Pike (nice guy) and explain your intentions. Infact if you can organise to meet with him (and show him a pocket bike) that would work way better than a phone call. The staging lanes would be the ideal location (better than the carpark)
Other venues... have you tried the AHG driver training centre? They have two skidpans and a sprint circuit. However they race there on Sundays.
There's the Wanneroo Go Kart track, didn't they used to race pocket bikes there anyway? Next to that track there's a massive skid pan (was used by MC motorsport before AHG driver training centre started) Throw out some witches hats and you can have two or three different courses in one day.
You could try somewhere in Henderson or Kwinana. There's lots of hardstands and asphalt areas around there.
Gateways shopping centre (in Success) has killer huge carparks/bus depot's, etc.
What about trying to organise something with an existing kart track? there's a few around... O'Connor indoor kart hire, etc.
If a well written proposal is submitted to <whatever council> you will probably have some luck getting a baskball court, school carpark, or something. You would need to pitch it as a "family fun event".