Sheldon,
I operated for many years out of Penfield. On quite a few occasions, till ATC banned it, I got a clearance from the Sunbury water tanks to Tulla to Essendon to Westgate or somewhere down that way. Hardly ever had any trouble.
First point: I always put in a plan.
Second point: I always made a taxy report to Melbourne Centre on 135.7 with 'expect clearance request airborne'.
Centre would invariably give me a code and 'remain OCTA'. That way, when I get airborne I can immediately be identified. In a surveillance environment you're very unlikely indeed to get a clearance until you are identified, which is why you probably won't get a clearance on the ground.
I would get airborne in the circuit or head out to the west a bit and call Centre for the clearance. They would usually tell me to call the Tower for a clearance having already coordinated the fact that I was taxying. If you were going the other way, it would probably be Departures, but same same.
As I said, hardly ever had a problem.
Philthy