Ok...

All the politically correctiveness aside... I'm a Canadian and I work for a US airline. It is possible... but obviously you have to go through all the legal avenues which is tough (and it should be). On the flip-side, Canadian carriers hire Americans and other citizens as well, who also have to go through all the legal avenues for work permits, etc. (also not easy). I cannot comment on other countries because I have no first-hand experience.