Its sort of like an extension bike rack idea? If you look at that concept all you need to ensure is your number plate is visible, and if travelling at night, probably lit up, brake and indicators could be replicated like a trailer with a flying lead and plug. I guess you should sketch it up and seek comment from either Vicroads or a VASS engineer. A Vass engineer should be able to advise if it legal to do it and what if any regs need to be complied with.