(Spoilers all) Melisandre and the Fingerbones of Davos Seaworth

In Melisandre's POV chapter she explains to Jon And Mance a little about the strength of her glamors.

"The bones help. The bones remember. The strongest glamors are built of such things. A dead man's boots, a hank of hair, a bag of fingerbones."

It strikes me as curious that she would choose a bag of bones as one of her few examples unless she had already at least put some thought into making that particular glamor. Could she have somehow found Davos' fingerbones? To what end would she use someone disguised as Davos? Who would she disguise? Possibly Devan? Tinfoily for sure, but I thought it was kind of interesting.