I used a texture that replaces U_B_CombatUniform_mcam. I disabled the cop clothing shop and removed everyone's cop_gear.
In core\config\fn_copDefault.sqf I set the cop's uniform to "U_B_CombatUniform_mcam".
In core\fn_initCop.sqf I added:
player setObjectTextureGlobal [0, "textures\mytexture.paa"];
before
[] call life_fnc_spawnMenu;
And then in core\jip.sqf I added at the end:
{
if (side _x == west) then {
_x setObjectTexture [0, "textures\mytexture.paa"];
};
} foreach playableUnits;
It's surely not perfect, and I have no idea if the loop in jip.sqf is superfluous (I don't know if setObjectTextureGlobal handles JIP by default).
Generally, I just tell my cops not to change their uniform at all and it works fine.