Seriously, refrain from asking her to dress alike..it's a bit weird to ask that of someone who is NOT your Long Time gf, your wife. To me that would mean the guy is looking for some way to put his mark on me (like dogs mark their territory), to show the world we're a pair (which is not the case if this girl ISN'T your gf already). I'd be wondering why he needed that.
I don't know, if I were you, I'd steer clear from such fetishes for a long time or I;d try to understand why I need them:) Do you care about being with her or your fetish?

Your logical explenation of my fettish seem to be accurate. Maybe subconsiously am trying to mark my territory.