A Yarmulka is a minhag, so I do not think it is a davar shel bkdusha. To prove, we kiss tzitis. We kiss Tefelin. We kiss a siddur. Do we kiss a yarmulka when it falls on the floor? Can we remove tzitis when wearing a four corner beged when going to work? We can remove a yarmulka.
So there is no issue putting ANYTHING on a yarmulka. If you want to argue ‘Se Pasht Nish’…that is a totally different argument.