As Commuter pointed out, "blocking" the seat next to you is a hidden previlidge of MPC, not so known by the public. (So let's keep it to ourselves ) On the 747, it doesn't make too much of a difference, because if you are seated on the window seat, you still have to climb out the last pax on asile. Also, the row 66-68 are almost guranteed full, so you will almost "never" get an empty seat if you are seated there.

On their airbus fleet, however, having an empty seat will greatly change your flight experience. My previous flights to Australia and back were always accomodated by an empty seat next to me, and I have to say it's just as comfortable as it can be in the Y class cabin. In addition, I would prefer to have an empty seat next to me rather than bigger leg space, because it gives me sense of privacy.

What you have to do, is to call MPC and simply request "Could I have the seat next to me blocked/empty". Some agents might be really nasty and say "we don't do that". If you hold status with MPC, probably they would accomodate you, but if they refuse, simply hang up, and call to another agent (probably at the different shift).

You can do so at the time of check-in, simply ask if the seat is occupied... HOWEVER, to do so, you will need to check in with the senior ground agent who are usually located at Biz/First class counters.

Enjoy.