My current problem with Steam is that it's always running and downloading data, and I'm on restricted internet. I try to use it in offline mode, but then ED won't boot for whatever reason. It's probably Frontier's fault, LOL.
There's a global as well as a per-game setting to allow/disallow background downloads while you're playing a game. By default, it should be set to disallow background downloads. You can see it easily by starting to install a game or having a game being updated and launching a game. The downloads should automatically pause, and only resume when you exit your current game. Auto-update of games can also be globally set and modified on a per-game basis.
So in theory, while you're playing ED, Steam shouldn't use much traffic if any (there's tiny stuff like updating achievements I guess, but ED doesn't have any). If it does (store update? friend list status updates?), well, you might be stuck if the specific feature can't be disabled. I'd be curious to know what that traffic is.
I'm surprised the ED client doesn't work in offline mode, considering it has nothing to do with Steamworks. The only difference I ever noticed between the standalone and Steam is the update process: standalone launcher downloads its updates (very very very slowly), steam launcher doesn't (game gets updated through standard Steam updates).
Did you have to register a Frontier account? If not, then it's a case of authentication being done through Steamworks. I'll try to launch in offline mode tonight, you got me curious.
edit: have a look here too in case something can help:
http://wiki.tesnexus.com/index.php/Make_Steam_friendly_to_low_bandwidth_connections