Maybe something like:
```csharp
// Cached somewhere else (maybe OnWorldLoaded) so you only need to do this once
var chatTraits = world.WorldActor.TraitsImplementing<INotifyChat>();

// For each chat line
if (chatTraits.All(t => t.OnChat(player, order.TargetString)))
    Game.AddChatLine(client.Color.RGB, client.Name + suffix, order.TargetString);
```