It's really hard not to give appearance-based compliments to girls, but I notice most people don't give them to boys past a certain age, or do it in a very different way ("You look like such a little gentleman!" "You're going to break some hearts!") I think it's worth thinking about that, and subverting it if possible: telling boys they're beautiful; calling a dressed up girl a 'queen' instead of a 'princess' -- or heck, let's get crazy and go with 'international superspy', 'ambassador', or even 'senator' or 'admiral', thank you, Star Wars! (And thank you so much for making sure my favorite princess growing up also wore a snowsuit and carried a big gun.) Girls don't sacrifice their potentials by enjoying dressing up. Boys are beautiful, and should hear it. And if they wrinkle their noses, they've already learned some things that should be unpacked and talked about.
But even past giving more creative compliments to looks...if one pays attention around a little girl one finds that she is being bombarded with assessments based on looks and femininity. Compliments are also assessments: they are telling kids what we value, what about them they should value too. Kids are sponges, and they pick up subtext as well as text. So even if a little girl is SHOCKINGLY beautiful, I think it's good to consider being the person who asks about her interests, compliments her drawings, or her impressive racing-car sound effects, because she hears about her looks all the time, and it's great for her to learn other values too. And that's without getting into the entire 'innate versus effort' compliment field of study, which is waaaaay off the topic of the thread so I'll stop babbling.
As to the original post...I hate the fact that waving around my committed relationship is the best way to get interested men to back off, too. I hate feeling like I'm supporting that premise you mention, the ownership thing, by relying on it. But as you say, there is an innate threat, if not of overt violence, then at least of rage. Heck, I've gotten cussed at for whipping out the 'I have a boyfriend' shield too early, when the interest hadn't yet been undeniably explicit. Because then I'm a 'stuck up B'. Which is, of course, also a failure of femininity -- one of the metrics of intrinsic worth on which I'm constantly being assessed by random strangers or new acquaintances!