Not sure if you are serious or just teasing but then wouldn't your partner be responsible and thus susceptible to the full extent of the law and be fined?
I don't think you can avoid paying rego.
edit:
example: A lot of times i stay over my friends place from a big night and get parked in by my other mates. Usually I just take one off their keys and take the closest car outside to do what I need to do and then come back and collect my own car when everyone is sober and awake.
Now, I drive my mates car and get pulled over. Car is unregistered and I claim "not my car" then so be it. More than likely since the car is unregistered, wouldn't the police stop me from driving the car? That is, I would have to leave the car there, ring one off me other mates and get them to pick me up. Obviously the police can find out who the owner is, issue a fine and get it sent to their address.
Only burden to me is having to get a taxi or get someone to pick me up but my mate will be issued a fine and will have to pay rego unless he wants his car parked in the side of the road somewhere and get raided or burnt, whatever.
edit edit:
I reread what you said and I think I know what you are trying to say but I highly doubt you can get away with that excuse and not paying the rego very long. Cops will have it on record the car is unregistered and if they actually do their jobs, it would be followed up.