No backstage nudity for me, but at age 18, I was backstage crew for a local play. The actors all got dressed in private Men's and Woman's dressing rooms, but makeup and props were in a common back of the backstage area. One of the women was well endowed and her period costume had a scoop neckline, so there was some cleavage showing. Nothing vulgar or even daring, but to an 18 year old boob obsessed young man, she certainly caught my eye! I was very jealous of the guy doing makeup, as he had to use the general pancake makeup on all exposed skin. He politely asked her about going lower after doing her face and neck, so he had the privilege of powdering her décolletage.
The only other changing type nudity I've experienced was a good female friend (and future lover) changing clothes to go out. She asked me to turn away (I did) but the the arrangement of a mirror in the room inadvertently gave me a glimpse of her beautiful breasts as she changed her top. Just a second or two, but still a thrill!