I doubt very much she either could or the story would go down that path. Sayaka is most likely a very average magical girl, while Madoka has the potential to be big.
Anyway, I don't see why Ryll is so bent on calling all the wishes very selfish. Because both Mami and Homura empasized it would be so? No matter what you say, Sayaka healing the dude's hand is not selfish. Mami telling her it would be was just basic psychology to make Sayaka think twice or thrice before making a contract. "He will certainly be grateful if I get the hand healed - Oh, no! That's an evil thought!" is the most ridiculous thing you could come up with. Which one is a bigger deal: The guy getting his dreams back or owing thanks to the girl (not that he would even know who did it)? And what is evil in trying to win the guy she obviously loves by helping him? Following that logic bringing flowers or chocolate to the girl you like is also evil and self-centered...
Bah. We haven't seen many wishes yet but until we see something genuinely wicked, it's much too early to call anything selfish.