T4 - Qatar won't allow OW elites into their London or Doha lounges, so no go there. I think MH uses a contract lounge in London.
KUL - not sure if the new satellite lounge is open yet (and I don't think the F side is yet either; a shame because the restaurant there was quite nice). The Golden lounge in the main terminal at KUL is fine - I don't like the showers too much but the laksa bar is good. Might be worth checking out CX if you have that much time; its not the nicest CX outstation lounge but is decent. MH means well but the experience is a step down in my opinion from other OW carriers, especially CX in HKG. Service is usually good though and the food can be quite tasty (if not the most premium looking)
On the way back, you should hopefully be able to experience the new satellite Platinum lounge for OWEs which should have new showers and the sit down restaurant back. Renderings look nice. You can see more about this in the MH forum. In Sydney the no brainer lounge is QF F. Enjoy.