Were it not a direct reference to real stage costuming — and were Yuu not mentioned as being very much present in Ayumu's earlier comment — I might agree.
I think there could be some transference going on; part of why Setsuna and Ayumu both like the tip-highlight thing so much does seem to be because Yuu does it, and I could easily see this Setsuna wanting that as a way to be closer to what Ayumu wants, or what she thinks she wants.
(I really love the later Setsuna bond episodes in SIFAS, when she's able to, well, come out to her family and be understood by them. This story would presumably be before that, but I can see her really treasuring this sort of hidden reminder of people who really see her.)
(And yes, I'm suspecting the "Nakagawa Nana is a closeted school idol" storyline has an intended sub rosa queer reading, if for nothing else for the fact that one of her post-coming-out stories involves her buying doujinshi with Riko.)
