You would also need alternate images since it is required that all rules exist on the actual map image and within the map size guidelines.
But this requirement could be changed for custom XML:s. Maybe an option in the game finder that you need to check to see the custom XML games, and also not making them visible in the "join a game" link. This would ensure inexperienced players wouldn't join the custom games unknowingly. Then you could just show a text message with the map that shows all the rule changes that come from the custom XML.