Draw and code all possible variants, make sure the whole thing is completely modular and compatible with the game's code, and implement it in a way that makes it feel like it was always there.
Sound like a lot of work for something insignificant? Because it is.
I know that Etheryn is a companion, but you have the option to give her transformatives to cause her breasts and butt, and doing so will modify her bust, both clothed and unclothed. Presumably, when you finish the Winter City, there's going to be variations of all of those busts (aside from the clothed ones) so that the chastity cage is gone, and since the author does say that they're thinking of letting us give her transformatives to enhance her "log," there's also going to be variations of her busts to reflect that.
Yes, Etheryn is a companion, and Sugo is just one of the characters in the town. But, they've already done stuff for variants, and there wasn't a promise made to the player to let them enhance her breasts and hips through a quest or even just dialogue or thoughts (at least, I think), yet the player can do it anyways (and I always do it for each character). Sugo, meanwhile, has an entire quest about getting Sugo a new caparison, and while, looking at the wiki, it could have just been done for 200 EC (my current character has, like, 13000 EC), I didn't know that since you need to talk to Leorah first about the caparison and then reject her deal (after talking to Ogrish you're led to believe she's the only one in town who can get Sugo the caparison) in order for the NPC who lets you pay 200 EC to show up, and the Metal Parts took an extremely long amount of time to farm. Then, you're told to pick the caparison's color, rather than it just being a predetermined color, which makes the choice of the caparison's color and design more personal to the PC/player.
It is, to the game's overall story, insignificant. However, lots of things are insignificant to the game's story: Shar has no relation to the story, beyond the player doing a sidequest which is just beating one single enemy which unlocks interactions with her. Yet, she's a character we (likely) bond with and care about, and she's got multiple busts (albeit, less than what Sugo would have with the caparisons). Her pre-pregnancy bust is different from her post-pregnancy bust, despite how they could have just made one bust for her when she's not pregnant, regardless of if she was previously, and one bust for when she is pregnant. It's insignificant, but it's still of import for those who have interacted with the characters enough that they'd get to a point where these potential bust variation may be.