well theoretically, yes its still possible. i picked this up again a few days ago.
i did a few of the single player missions on hard that i hadn't finished to unlock the CFA44 jet. Then I went Online for some team games. to my surprise i got a game instantly with about 10 players in it. however, the first game was full of serious pros, i got absolutely caned (though i am not crap by any means), all but one member of the team i was on quit after a couple of minutes. i managed to shoot down two planes, but died loads and loads.
i then tried again, but just kept getting battered, i ended up with about 40 points, whilst the enemy team ace had over 27000. the other team obviously new what they were doing as they had varied planes with a tornado and a ECM pod or whatever. so i scored only one kill.
i suspect the team i was up against were a team who play together and are using the premium DLC versions of the jets, thats just no way they were flying that much better than me, i was constantly getting missile warnings, they just kept firing and firing. it wasn't much fun.
so if you come up against that lot, your unlikely to get the multiplayer kills achievements, unless, you use the DLC planes. i may get one, i haven't decided yet though.
if you fancy doing some of the co-op missions, you can send me a FR,as there was no-one on that mode at the time.