Although this method doesn't give you a playable nude character, it will allow you to spawn Yennefer & Triss among others and make their appearance naked.
It's really simple, all you need to do is go to Debug Console Enabler at The Witcher 3 Nexus - Mods and community and download the Debug Console Enabler, which is basically what CDProjektRed uses when they are working on the game and test things. However, with the console you can do a lot of things and one of these things is to spawn character models and sift through their alternate appearances.
Install the mod by following the instructions on the mod page, basically just a drag and drop job. When in-game press F2 or tilde key to open console and type "spawn(yennefer)" or "spawn(triss)" for example without the quotation marks., hit Enter. They should now be spawned. Go out of console again by pressing F2 and "aim" the camera on the character model you wanna alter the appearance of, then re-open console and type "appearance(yennefer_naked)" for example and hit Enter. With F1 you can go into free camera mode and have a look at the beautiful landscape or other things that might interest you more ;)))))
Made a little demonstration video here (still processing as I'm posting this, give it a little time). I use a XboxOne controller for smoother camera movements, and I also found out I could zoom in and out using the D-pad on the controller so it serves better in that regard compared to kb+m when it comes to free camera movements.
https://www.youtube.com/watch?v=AJGNm2q2KQI
The console commands list are still growing and new are being discovered as we speak, check on the nexus mod page for an updated list of all known console command list that will enable you to do tons of things in the game.
PS. Yes, you can use the appearance(yennefer_naked) command on the default/standard model in the game and it should carry over into cutscenes and conversation as well, although sometimes it resets, but then just reapply.
PPS. When in free-camera mode the game doesn't actually pause so all the things you're normally able to do with Geralt you will also do while in free-camera mode.
