Thanks to A69 (*8052) and flt001 (*8068) for posting the link to the AAIB data.
A lot of posters have stated that the autopilot in TRK mode would follow a constant magnetic heading.
The geomagnetic South pole is now at about 80 S, 107.5 E.
Magnetic Poles
If the heading had been set to due South (magnetic), that would seem to give a track somewhere between the 400 KTAS and 450 KTAS curves on the last AAIB figure:
https://www.facebook.com/17856688885...614509/?type=1
MAGVAR is about zero over Kuala Lumpur, and increases to around 20 E at the last positions shown on the AAIB routes. For MH 180, that would favor the 450 KTAS line.
Historical Magnetic Declination | ngdc.noaa.gov