I added different hair types in the game; straight, wavy and curly. They work just fine, but they're random just like everything else and I'd like to add to character creation a possibility to choose a hair type for your character (and subsequently for your starting slave, as they're the same menu). So, what I need is to add either a drop menu or text box to stage 6 in character creation. All the mechanics seem to be in mainmenu.gd but I can't find the portion where the actual layout of the menu is set, or designated file for it.

Any help? I'm fairly certain the hair styles are fully working in the game. Only place where they need to be chosen from is character creation. It doesn't need to be an option in slave tab or even at slave services. They can be edited with the Mutation spell, and that's enough for me.

I assume your hair types are different from hair styles, which are accessible in the slave customization tab. If so,  mainmenu.scn is the file that dictates the GUI structure used by mainmenu.gd. It is not a text file, so you will need to use the Godot editor to change it directly. You can also use code to change the GUI structure, but that can be less intuitive to many people. The Constants mod and Improved Random Portraits mod both use code to add their GUI controls.

Yes, they are different. It bothered me how hair could be wavy only at certain length, and how open hair was always straight. Minor detail, I know, but I tend to be quite catastrophic combination of neurotic , obsessive and perfectionist...

Thanks for your answer, now I know why I had such trouble locating the GUI structure. I guess I need to take a look at Constants mod then. 

Actually, I just downloaded this Godot-thing. It was quite intimidating and confusing at first, but the job is now done. Interesting tool.