We already have a fully functional scenario editor, that doesn't appear very far off of creating multiplayer scenarios. Adding something that define who the players are wouldn't be that difficult with the elegant and simple scenario format they have.
As far as whether they should affect rating, I don't think they should at all. They are a totally different game, as they generally focus entirely on the tactics of a given situation. And there could be some pretty interesting situations
Another thing is that every scenario will be different. Everyone has their strengths and weaknesses, and since a scenario is limited on what skills it focuses on, some people will be better at some scenarios, but horrible at others. It would definately be good practice to focus on ones that you need work on, but I don't think it should affect your rating. Besides, how is it possible to base a rating on how you are at multiplayer scenarios when each one is so different?
If the scenarios are well balanced and well tested, perhaps a rating system might work. I would say if anything that it should require a seperate scenario rating though. So a player would have his standard multiplayer rating, and also a scenario rating.
Just my initial thoughts.