You can have your superannuation paid out to whoever you want, you just need to nominate your beneficiary. If such a contract was entered into, the beneficiary would have to be changed, or the contract would be breached when the superannuation vested. If you are still listed as the beneficiary, superannuation providers will not pay your entitlement out to anyone else with anything short of a court order, and whether or not a court would be willing to make a decree of specific performance in such a case is doubtful.
In short: I don't see why such an arrangement would be illegal. The Family Court has something similar called 'flagging orders' which are used to split super, but not have the entitlement paid until it vests. However, unless the correct arrangements have been made, enforcement could be impossible/costly, and/or result in large damage orders being made against you.
Also, what Frigid said.
Edit: Any question of illegality could easily be solved by employing a legal fiction, whereby you keep your superannuation entitlement, but pay the other person a certain amount "out of your own pocket". Obviously, you are reimbursed when your super vests, and the other party would still recieve their money.
All of the above arrangements would only be effective if you have a super fund which allows you access to the full measure of funds as soon as it vests.