I've been working on a version of the mod that replaces any hat/misc that disables a bodygroup with the model of the bodygroup it disables. It's slow progress, but so far it's working nearly perfectly on sv_pure 2 servers for the few hats I've replaced so far.
The one problem I have is with textures for bodygroups with team colours (the soldiers helmet for example). Somehow, when a model is replaced the game defaults to the texture that the model uses, which has saved my bacon from having to retexture every hat that replaces a body group (and create entirely new textures for hats that disable multiple bodygroups eg. Backwards Ballcap with Headphones Disengaged style active). However, the bodygroup models with team coloured textures only use the red texture. I'm pretty sure it's an easy fix, I just need to find the textures for bodygroups first. Does anyone have any clue where these might be located? tf2_textures_dir.vpk > materials > models > player > items > [class] only has the textures for parts of the class models that aren't bodygroups.
Google searches have only given results relevant to the old .gcf files which seem to use a different file structure (one result said point to tf2_materials.gfc > tf but there is no 'tf' subfolder in tf2_textures_dir.vpk)
Bonus points for anyone who can find the .phy files for the default hats.
Edit: Well, fucktits. Turns out the textures for bodygroups are included with the texture for the rest of the model, face, body and all. I have no clue how I'm going to fix this. If anyone is savvy with making custom hats/skins/anything with TF2/The Source Engine/.qc files, feel free to add me and I'll walk you through the process of replacing the cosmetic model with the body group model in the hopes you can figure out how to change which skin is selected.
Also, I apologize for my poor job of explaining my problem. It's much easier if I walk you through each step so as I said, if you think you might be able to help, don't hesitate to add me.
Edit 2: I got the retexture to work by changing which .vmt is referenced in the .qc file, however sv_pure 2 spits the dummy at this point and the hat reverts to it's default model. Assuming Valve never allows custom textures for hat models by whitelisting them or something, we'll have to chose between Soldier hats that disable the helmet bodygroup being visable on sv_pure 2 servers, or having the blue soldier wearing a red soldier's helmet on any server. I'll try to email Valve to see if they'll change this, but getting them to change a huge part of sv_pure seems like a long shot. I'll continue to work on the mod with blue soldiers wearing red helmets in the mean time.