There is a disability travel section that has several threads that discusses cpap usage inflight here:
http://www.flyertalk.com/forum/disability-travel-224/.
I regularly fly transPacific and use it all the time. Best way to check is to see what their website says re medical devices, second option is to actually call them and ask. Airlines that I've used in-flight with on-plane power includes Eva Air, Cathay Pacific, United Airlines, ANA and JAL. No experience with European carriers.