Which aircraft still operate with a ring main system?
Certainly some (?all) 744s, certainly some (?all) 777s operate a ring main system (though some T7s I know of use generators in one of the cabin crew crew rest areas).
Wiggy, in my old outfit it was forbidden to tap into the ring main for planned medical cases. The Stretcher Kit came complete with sufficient Therapeutic Bottles for the whole trip.
Looks like must vary from operator to operator - FWIW where I am we are certainly allowed to tap into the ring main for medical cases if it looks like the passenger is going to need supplementary oxygen for an extended period, regardless of whether it's a pre-notified case or the crew are dealing with somebody who has fallen ill on the flight.