I finally got around to call Cathay about my possible Malaysia Airlines flight from KUL to SIN.

They told me only the following fare classes are eligible for membership extension.

Economy: YVHKM
Business: FAPJCDZ

What irks me is that Malaysia Airlines economy glass is only Q and N at the moment, with N being their economy flex.

So it looks like I'm out of luck with Malaysia Airlines unless I book a business Z class