[Lucifer, you'll... you'll have to make your innuendo less modern if you want Sebastian to fully get what image you're evoking. As it is, he figures it is indeed some form of fetish (because what isn't, with Lucifer?). Once again questions the appeal of maids...]
You guess correctly in the sense that he has a charge already. [If you want to phrase that as him playing dress-up as a maid, fine...]
["Charge" is a very pretty way of saying "someone he probably has his sights on eating".]