Assuming she was able to procure Davos' lucky bag between the Battle of Blackwater and Stannis' attack at the Wall, Melisandre may have already cast that glamor. However, something else stood out to me on a reread of A Storm of Swords, in Davos III, where Davos is lying sick in his cell:
We know from Arya's time with Roose Bolton at the very least, that leech therapy was uncommon. However, Roose was awake for his leeching. The other time we see a leeching done on Dragonstone is with Edric Storm, and lo and behold who was the "leecher" for that:
We have evidence that Pylos is working with Melisandre, to bring her blood! Davos' blood cannot be considered "kings blood," so why would she want it? It seems reasonable that she will use it for a powerful Davos glamor. Perhaps she extracted his blood for this purpose because she hadn't gotten the bag of fingerbones yet.
The question is not whether Melisandre has planned to use a Davos glamor; there is sufficient evidence that she has planned this since at least Dragonstone. The question is whether she has done so already, or whether she will do so in the future. My prediction is that Jon Snow will be glamored to get him out of Castle Black, but that is neither here nor there. There will be a Davos glamor showing his face, though, and that is worth thinking about.