Asked the Labuan agent, and she said that I am indeed entitled to an extra 20KG of baggage. Today at Miri checking in for a DCH6 flight to Mukah however was told that this only applies to MH mainline flights only. This was at the J class counter where I was checked in without any problems. Neither airport has a lounge, so can't say if they would allow access in reality.

I have one more sector with Maswings soon and a Firefly one as well. Firefly bookings don't even show up on the MH website as they seem to have a separate system.