Over on the RPGSite, there's a thread by Ryan Dancey about how games need to evolve with more rules about relationships and similar things, and less emphasis on combat. While I don't agree completely with this idea, I think a way to reward character relationships that I could tack onto S&W or BFRPG is a good idea.
Some generic ideas:
1. PCs get XP rewards if they work on a relationship with an NPC. This would need to be something important, like a spouse or a major contact. (Taking the princess to a ball and dancing the night away, on your own initiative).
2. PCs who actively work towards a relationship goal (marry the princess, become a confidant of the king) might get a bonus. Maybe as long as they are working towards the goal, they get +1 to rolls directly involved with achieving it.
3. PCs who ignore a relationship might gain a penalty, such as a reaction roll penalty, while dealing with the NPC and his friends. (If you blow off the princess after she asks about you, she gets angry).
4. PCs might be coerced by a significant other to accomplish some goal, by receiving a penalty if they ignore their relationship, and only losing the penalty when they start working on it. (If the princess invites you to a soiree, and you avoid it for some reason, you receive a penalty until you actually meet with her).
Not sure if I would use these or not, and if I did, I wouldn't use all of them. But I think it's an idea worth exploring.