That's actually a good point. I travel with a CPAP and do use it on board (including recently on a SQ flight before these rules came about). Most airlines don't require any clearance at all to use a CPAP (they do warn that inflight power often can't support the operation of a CPAP); QF and JL are amongst some notable exceptions where clearance must be sought prior to the use in flight (specifically for the machine, not the battery).
Surely SQ et al would allow battery bank use for medical devices, perhaps needing to apply for paperwork in advance of flight. My CPAP battery can also be used to charge USB devices, but I suspect that would fall outside the scope of acceptable (excepted) use.
Something that also isn't all that clear is whether these rules are as a result of local laws (e.g. instituted by the host country of the airline) or merely a policy of the airline. If it is the former, surely all airlines flying to and from said jurisdiction would be subject to the same kinds of rules and restrictions.
