I believe Shireen's role in the story has more to do with her greyscale than being any sort of possible sacrifice for Melisandre. ADWD really emphasizes the condition, both with how it's viewed by Val and the Wildlings, and the stone men during Tyrion's voyage east. Perhaps the White Walkers can control those afflicted with greyscale, much like how they control the dead. Would explain why Wildlings fear it so much and kill anyone who is infected. Ever since Shireen was cast, I figured she had some important role to play down the line that we just haven't read about yet. She's a character that could've been easily cut completely from the show, or at least not sail north with Stannis, but obviously D&D kept her in for a reason.
There was also some talk of how Craster dealt with his greyscale daughters that piqued Sam's interest in this episode, and it sounded like Craster offered them up to the White Walkers just like he did with male babies.