To the OP - call MPC and ask what is the cheapest full mileage earning fare that CX is selling between xxx and xxx ?
MPC: L class
Then please ask them to book in that class.
MPC: L class is sold out, sir.
I'm MPC Gold...
MPC: Okay, yes of course.
There - job done.
You cannot book online when there is no seat available, so in order to use the seat guarantee, you need to book through either a TA or MPC..