Most men's braces...the one's that aren't clip-ons...have a double button-hole arrangement in both sides of the front. Hard to describe but people familiar with good men's braces probably know what I mean.

So...why not sew a button on the inner side of the inner apron and another button (for the same strap) on the inner side of the outer apron. Do the same for the other side of the front of the kilt. In the back you would sew both buttons on the inner side of the kilt. What I'm getting at is...wouldn't that keep the aprons even?

I am going to sew buttons on my kilt to wear with a waistcoat simply because I have the same problem--no hips/butt to speak of. I've seen too many old (vintage) photographs of kilts with braces to think it odd. And if buttons are used and sewn inside the kilt, the kilt can still be worn with a belt and no one will be the wiser.