I feel like a complete idiot. Mesh won't show in sfs again.

Hi. You need to assign cut numbers that match with those of the original meshgroups. You should also watch tutorials on converting or adapting meshes for ts4 because none of it is straight-forward

You need to redo everything because the uvs are messed up:
- clone a hairstyle of similar length, export the blend, open it in 2.76-2.79 version of blender;
- delete the original 3 meshgroups;
- import your obj, do not edit it in object mode ;
- switch to edit mode and edit its shape if you need to;
- in edit mode add a new vertex group, rename it to b__Head__, select the hair and assign it to it;

- rename a uv map to uv_0, set the background, shrink the uvs to bring them back to the original size, put them in the space meant for hair;
- create a second uv map, name it uv_1;
- unfold rig, make head_2 visible and selectable. You hair goes in the upper body area, so normally you'd want to duplicate the head and the top, join them and use it as a reference, but since it goes only a little bit, i think it's fine to just transfer from the head. Anyway, select the hair, add Data transfer modifier, head_2 as Source object, set the rest like in the pic, apply;

- vertex paint the mesh with 007f00;
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- in object mode duplicate the mesh three times, assign cut number 0000, 0001, 0002