Who knows, maybe it is possible, but I just think it is not. But when it comes to basic things, such as "collect 50 bread", I would add that adding this message may not be necessary at all. The one who plays the scenario would check the objectives, and they would know what they need to do. Not perfect, but just saying. Still, for more detailed answers we should wait for others, such as Doomsword to tell us what he knows.
Regarding changing of the default lord, it is possible to change the default lord in entire game - by switching the body files for different lords in the Stronghold 2 folder. There is also a way to change the default keep for kingmaker games, so if you like - you may be able to start all your games with wooden, small, or even huge keep. Though that doesn't have to do much with changing such details in one specific game.
Again, Doomsword knows a lot about these things - from map making and scenario creating to modding the game. Hopefully he will notice this thread soon, and tell us what he knows.