Such an interesting question, Gryffin... (and congratulations to your son, by the way!)
In thinking on this, it sounds as if there are several elements going on...there are issues of class signalling...which you allude to...and also, perhaps, the soloist's response or attempt to respond to what I perceive as a subtle or not-so-subtle "sexualization" of female performers, especially soloists, in classical music. Was she trying to be "sexier" than the others (those slits, those red-soled stilettos!), or was she trying (with a jacket and pants) to masculinize herself compared to the usual for female soloists (who often appear in gowns in other contexts)...or, oh, who knows?
If she was trying to be "sexier" in some way, it's understandable, given the pressures on women to do just that, and the incredibly competitive nature of the field...yet at the same time, what a delicate dance this is for women, for if she appears to be "trying too hard" she will be judged by peers and mentors/ public alike as over the top.
I can't help feeling that if SHE were HE, the situation would be so much easier. Even if his clothes were to the trained eye more expensive than his peers.'
The moral of the story, to me, is that it's just plain difficult to be a woman performer!
It's a really interesting question for me as someone who sometimes has to appear in the public eye. I find "appearance" dressing as a writer to be extremely difficult because there is no uniform. My own solution is, essentially, to create my own uniform. For most occasions, I'll wear a navy blazer, jeans -- to signal informality -- or trousers if the event is more formal -- some kind of bootie or sandal, depending on weather, some kind of top -- varies according to what I perceive as the formality of the event. I never feel as "cool" as my black-leather moto clad rocker-chic style colleagues, or as "arty" as some of my EF clad colleagues, but it's a bit truer to who I am. Still, there are times when I still feel I don't have the "right" outfit or perhaps the formula needs some tweaks to feel less conservative. Sorry to derail there -- just thinking on the keys.