I encounter this stiuation a lot.
Usually I phone MPC to book the outbound itinerary when it is just open, and then inform the agent my inbound flights would be in 3 months(or whatever period).
My outbound seats could be held 3 months and the deadline to issue the ticket postpones.
However, the outbound flights must be CX/KA exclusively. MPC couldnt hold any partner airline reward seats.