I tried the "call for pre-clearance" policy on a recent flight in first class from London to Hong Kong.

The good:
I used my CPAP machine inflight without being accosted by the flight crew. I plugged in and had a great snooze without incident.

The bad:
Getting clearance was a painful ordeal.
My journey to on-board sleep started when I called CX reservations and spent close to 30 min on hold. The agent who eventually answered had absolutely no idea what I was talking about and had to speak to a supervisor. Another 10 minutes on hold. Then the agent came back asking me to spell out CPAP in long form. He then disappeared for another 10 minutes and the call dropped. 50 minutes on the bloody phone and still no further along. I thought they may call me back or send me an email, but nothing.

I called again and waited 20 minutes before giving up.

The next morning I called in again and waited 15 minutes before getting an agent. Again, no idea what I was talking about and no record of my call the day before. Better prepared this time, I helped the agent navigate to the spot on the CX website where they publish CPAP information. Back on hold for 10 minutes, agent back on the line to ask more questions, back on hold for 10 more minutes, more questions. Finally the agent came on and asked if I could use a battery of if I had to plug in to the aircraft power. I don't have a battery, so I said power. He replied that if I told him I would use a battery, he could approve. So I did. (very Hong Kong way of approaching the rules).