This modders resource adds one non-replacing demo hairstyle through Content Patcher.

I felt too many mods replace hairstyles or append new hairstyles to the end of the hairstyles image, in a way that having multiple hair packs would conflict.
This mod uses a SDV 1.5 feature that allows writing into the Data/HairData asset to define custom hairstyles.
With this method, modders can add as many hairstyles as they want and their mods will not conflict with each other.
No hairstyles.png overwriting necessary. No Get Glam necessary.

This mod is intended to be used as a modders resource for other mod-makers.

How it works
We insert a hairstyle spritesheet into the Characters/Farmer/ asset folder with CP.
It can be named anything as long as its consistent within our content.json.

Then we insert lines of data into the Data/HairData asset. Each line defines a new hairstyle.

Data/HairData uses the following format:
"100": "hairstyles2/0/0/true/-1/false",
The table name (100) is a unique ID. We need ours to be unique.
This ID cannot be larger than 2,147,483,647 (the maximum value of Int32)
A common trend with NPC events is to start with the last 4 digits of your NexusMods profile ID, so that is what I've done in this demo.

Then we have the following entries, separated by slashes:
0 - The filename for the file we added to Characters/Farmer/, without filetype.
1 - The tile position for our hairstyle sprite, x coordinate.
2 - The tile position for our hairstyle sprite, y coordinate.
3 - If we have a unique left sprite or not. If false, it uses the right hair flipped. If false, we need a fourth sprite.
4 - Either a unique ID of a separate hairstyle for when player is wearing a hat, or -1 to hide hair.
5 - "is bald style". Currently, just set false.
6 - unused
7 - unused
8 - unused

Notes
We're likely still limited to ~336 hairstyles with this method, but this is untested. Spacecore may solve this if it occurs.
Users have shown this mod supports more than 700+ hairstyles, so we are not limited to 336.

Credits
Art by Charles Gabriel. Slightly modified. Under CC-BY 3.0 - https://opengameart.org/content/16x18-rpg-characters-hair-clothing-pack

Changelog
v1.0.0 - Initial release