That's right RDK.
The metro taxi boundary really only defines where taxis can pick up UNBOOKED work (i.e off the street, at a taxi rank, outside a pub, etc). Metro taxis are allowed to travel outside the boundary to pick up pre-booked work, and to drop off passangers picked up from within the Metro zone.
Likewise a Frankston cab is allowed to travel to the Eastern suburbs to pick up a booking and take them to Sorrento - and he will have less distance to travel to get back to his working area too.
It wont be cheap though, especially travelling before 6am. Between Midnight and 6am Maxis can charge Tarrif 3 - I'm not sure what that is these days, but it would be well in excess of $2 a kilometre.
Your best bet is to either make a private booking with a Maxi driver (and be prepared to haggle - they are entitled to 'quote' the fare rather than meter it for 'out of area' work). It's a nice fare though, as the driver will very likely be at the start of his shift (beware of the drivers who are at the end of their shift at that time on Sunday morning - they will have already driven 500km in the company of drunks all night, and might be a little weary).
"It's life Jim...but not as we know it"