Greetings everyone,
Me and my girlfriend spent a few hours figuring this one out. If you've researched the topic at all, you know that the problem is caused by the lack of DirectPlay in the default install of Wine and the bottle.
The fix to getting the multiplayer working is to go to bottle management, select the bottle you've installed Baldur's Gate 2 into, and install new software.
From there, just search for DirectX9 (or DirectX Legacy on CrossOver 9 and earlier) and install it. After that the TCP/IP multiplayer mode started working for us.
Everything else works normally, except when you try to quit the multiplayer mode either by closing down the game, or by canceling the game setup. In that case the game hangs, but that's a minor nuisance when you can actually play your beloved game. The saved games also seem to work properly, and don't get corrupted even if the game hangs on exit.
I hope this information is useful. Have fun playing Baldur's Gate 2 with your friends :)
NOTE: The lady here reminded me that it's important that you don't try to install any patches from the web. Only install Throne of Bhaal if you got it. If you have any version differences, the multiplayer will refuse to work. The joining players simply drop from the game.