Can’t you have the same behaviour as multiplayer robot that any mods/addons are allowed when creating game.
The save game parser can extract the information found in the savegame and present it on the game-web-page so potential users can see what mods are required for the game.
Atleast allow so I can start a game with YnAMP - Yet (not) Another Maps Pack | CivFanatics Forums
Also allow change of number of players in the game when you have started, but not yet uploaded the first savegame.