The way I see it is;
If the new airline you are going to are asking you to bust your notice period then maybe they are not quite the 'great opportunity' not to miss. Not a great an attitude for a new employer.
Even Ryanair, yes shock horror, doesn't ask people to not work their full notice.
Lets face it most established airlines know its 3 months notice for the vast majority of pilots and has been for a long time.
Yes if you have months of unused leave etc but work your notice as the grass is not always greener regardless of the Ryanair debate.