That is a nice angle. And it seems he might have already done that to a point without realizing it, just by being him. When he realized he still had free will over Mab, and told her he would carry out orders exactly. That was Harry talking, putting the Mantle mask aside. The same as his birthday party in winter, when he killed those Fae. It wasn't so much the WK, but Harry Dresden utilizing the power of the Mantle without putting on the mask. Also, I remember a tiny bit from Fix, books ago, when he warned Harry about the leaf, Titania had given him. That was Fix, putting aside the mask and forcing through the Mantle's gag to warn Harry, when the Summer Knight would never have. Harry of course has power over Lashiel in a way as well, knowing her more than anyone alive that is still free of the coins. Even if he doesn't realize he has that power.