I was able to add additional content to the game and play multiplayer LAN by modifying the DLC. It should work for any multiplayer mode so long as all players DLC modification looks the same. The DLC folders are found in the game directory under the steam common apps, then Assets\DLC. I used the Babylon DLC (DLC_Deluxe).
It was really easy. I had made a mod that added eight civilizations to the game with icons, buildings, etc. All I had to do was copy the actual content folders within my mod (in this case I had one for each civilization) and edit the Babylon.Civ5Pkg with a text editor. All I added was art, text, leader, and civ, but you should also be able to add music and animations. Within the .Civ5Pkg file you will see where the content is added by type and not file. It simply looks for the content type in the file specified. For mine all I had to do was add this before the end of the file:
<GameData>Bearveria.xml</GameData>
<GameData>BearveriaBuildings.xml</GameData>
<GameData>BearveriaDiplomacy.xml</GameData>
<GameData>BearveriaGameText.xml</GameData>
<GameData>BearveriaIcons.xml</GameData>
<GameData>BearveriaPlayerColors.xml</GameData>
<GameData>BearveriaTraits.xml</GameData>
<GameData>BearveriaUnits.xml</GameData>
<GameData>Bearveria.xml</GameData>
<GameData>Nicholas.xml</GameData>
<TextData>Bearveria.xml</TextData>
<TextData>BearveriaBuildings.xml</TextData>
<TextData>BearveriaDiplomacy.xml</TextData>
<TextData>BearveriaGameText.xml</TextData>
<TextData>BearveriaIcons.xml</TextData>
<TextData>BearveriaPlayerColors.xml</TextData>
<TextData>BearveriaTraits.xml</TextData>
<TextData>BearveriaUnits.xml</TextData>
<TextData>Bearveria.xml</TextData>
<TextData>Nicholas.xml</TextData>
Bearveria was my civ, and Nicholas was the leader. My xml's also contained the supporting text references, so I just had to duplicate their use. Without doing that, I got the TXT KEY instead of the city names Bearopolis and Ursadelphia. I did not have to reference any of my .dds's in the file anywhere; the game loaded the art lickety-split.
I repeated this process for the seven other civs I have put into my CivV mod. I copied my DLC folder to my multiplayer partner's computer. We checked off Babylon on the DLC when setting up the multiplayer game and voilà, we could choose any of the eight civs. We each took one and added the other six as computer opponents. The game didn't balk. It took us right in, and we were playing immediately. This did not interfere with Babylon; we could still choose to play as them as well.
I do not believe this will work if you are looking to change any of the original content only add additional content. But, nothing ventured, nothing gained.
I am working off the patch (hotfix whatever it was with no change log!) that was just updated today, 17 February 2012.
Happy Civing!
Chazz