MacBook Pros do not "need" 85W of power. They run fine off the MacBook's 60W power supply, charging and all. They just come with a larger adapter because there's the potential for drawing more power, if you have a bunch of stuff attached and are keeping the CPU busy, etc.

I find that the MagSafe will, depending on the aircraft, very very slowly charge the battery. Once on CO AMS-EWR and CX AMS-HKG I had it charge by one or two percent over the duration of the flight. This is because the voltage on the EmPower ports is 15V whereas the adapter does 17.5V. With a regular socket, the amount of power available is normally around the same 75-ish Watts, but the adapter is of course the regular one so the battery will charge, though perhaps slightly slower than normally. I charged my battery from 50-ish percent to 100 on EC AMS-JFK just two days ago, and they use regular AC sockets (that luckily fit European plugs, the US plugs are ridiculously bad.)