I'd like to share my experience on using CPAP on CX.

I have applied to CX for a frequent medical card (they call it "freqmed card"). The model of the CPAP machine is pre-approved and everytime you make a reservation, you just have to reference it to your travel agent.

I have been using CPAP on CX flights for years (I am DM frequent flyer), and so far, have only seen one other person used it inflight. CPAP machines have become vastly smaller in recent years and operate much more quieter. They are certainly quieter than the engine noise, and it certainly stopped me snoring along the way

The HK check-in control is fully aware of what CPAP is (although some of the check-in attendants might not be familiar with it). Even on the occasions that my agent forgot to put in the registration, so long as your CPAP runs on battery and not connected to the electrical system of the plane, no one is bothered.

I use the Respironics M series with a Li-Ion battery cell that I bought online from a US supplier. Works great.