I don't think there's ever been an adult game that's had a large enough following to get modders who would do something like this for free.
Actually, there's been quite a few.
-Artificial Girl 3 by Illusion. Japanese game where you customise your own harem of girls and have sex with them. Not only have there been uncensor mods produced by western players to add realistic vagina and penis models/textures, but there have also been numerous texture mods for both the skin and clothing, especially on the clothing side. AG3 works by having very simplistic clothing pieces that can be mixed and matched to create different shapes of outfits, relying solely on textures/alpha maps for details. Thanks to this, there are 100s of mods recreating outfits of well known anime characters so that they can be recreated in the game.
Most games by Illusion get this treatment (texture mods), but AG3 is the best example.
-3D Custom Girl by TeaTime. Another Japanese game that is purely a simulator, much like 3D Sex Villa. There's no plot or anything of the sort, you simply create your girl and choose some poses/animations to play, or even create your own poses in the pose room. This game has a FUCKING HUGE modding community, especially on the Japanese side. Custom body models to appease all different sorts of fetishes, including a male body for twink/trap characters, custom hair models, custom face models, custom clothing models (as in, actual modelled clothes and not just texture edits) based on an array of characters, including TouHou, Sailor Moon, Final Fantasy, Air, etc, custom locations, custom animations, custom shaders, not to mention the ability to package and redistribute the poses you create in-game. The modding community is so large, the game has officially been classed as a sort of art program by TeaTime, allowing users to export images they create using 3DCG and using them in their own projects (kinda like Poser or Daz Studio). Heck, there's actually a small handful of RPG Maker VX games out there doing this exact thing, using 3DCG to produce the character art and porn scenes. There's even a plugin allowing you to export any 3DCG girl you create into a MMD (MikuMikuDance) character.
3DCG isn't the only TeaTime game to have some nifty mods made for it. One of their older games, Love Death 2, had some really nice custom models too, including one for Hatsune Miku.
-3D Sex Villa 2 by Thrixxx. Admittedly the official modding community is a bit drab, or so i've heard. The unofficial (technically illegal) modding community though is very active. I'm not going to mention the name for obvious reasons, but I am referring to an illegal modded version of 3D Sex Villa 2 that can be pirated off the net. This modded version includes every single character model, animation, and location ever seen in any of Thrixxx's games (based on the same engine as 3DSV2). This is notable in adding functionality that 3DSV2 previously lacked (such as m/m parings, and shemale characters). The engine itself has been hacked to include plenty of new customisation features (such as the ability to create taller or shorter characters), new textures, new outfits, and more. However, as nice as this is IT IS ILLEGAL, hence why i'm not sharing the name of the mod nor will I share a link to it. You'll have to find it yourself.
Point of the matter is, adult games DO have large modding communities. Heck, the non-adult ones like Skyrim actually BECOME adult games thanks to the modding community, with mods like SexLab adding in sex animations and controls to Skyrim.
If X Moon were to add even a basic modding toolkit for XStoryPlayer, at the least allowing for new skin textures, I can assure you that people will use it. The modding community for adult games is bigger than you think.