2D Outline shader issue: Apply outline on separate sprites as a whole?

Hi all,

so I have this 2d outline shader which is a modified version of the shader in the comment to this answer. The idea is to represent health in an outline, you start with full health = full outline = green. As hp goes down, the outline fill decreases and it becomes red.

The issue however is that I was testing it on a single sprite, when I came to the actual model that we're using, it turned out that it was a complex sprite made out of many renderers. Every piece acts on its own now :(

Please see this video demonstrating the problem.

Here's a simplified version of the shader without the "Fade to black" and "Fill inside" portions seen in the video, just the outline and thickness:

 Shader "Custom/2DOutline"
 {
     Properties
     {
         _MainTex ("Base (RGB)", 2D) = "white" {}
         _Width ("Thickness", Range(0, 1)) = 0.5
         _Fill ("Filling", Range(0 , 1)) = 1
     }
 
     SubShader
     {
         Lighting Off
         LOD 110
 
         CGPROGRAM
         #pragma surface surf Lambert alpha
 
         struct Input
         {
             float2 uv_MainTex;
             fixed4 color : COLOR;
         };
 
         sampler2D _MainTex;
         float _Width;
         float _Fill;
 
         void surf(Input IN, inout SurfaceOutput o)
         {
             if (IN.uv_MainTex.y > _Fill)
             {
                 fixed4 c = tex2D(_MainTex, IN.uv_MainTex) * IN.color;
                 o.Albedo = c.rgb;
                 o.Alpha = c.a;
             }
             else
             {
                 _Width /= 100;
 
                 fixed4 TempColor = tex2D(_MainTex, IN.uv_MainTex + float2(_Width, _Width))
                                  + tex2D(_MainTex, IN.uv_MainTex - float2(_Width, _Width));
 
                 fixed4 OutlineColor = 0;
                 OutlineColor.r = (1 - _Fill) / (_Fill * _Fill);
                 OutlineColor.g = _Fill;
                 OutlineColor.b = 0;
                 OutlineColor.a = 1;
 
                 fixed4 AlphaColor = (0, 0, 0, TempColor.a);
                 fixed4 MainColor = AlphaColor * OutlineColor.rgba;
                 fixed4 TexColor = tex2D(_MainTex, IN.uv_MainTex) * IN.color;
 
                 if(TexColor.a > 0.95)
                     MainColor = TexColor;
 
                 o.Albedo = MainColor.rgb;
                 o.Alpha = MainColor.a;
             }
         }
         ENDCG
     }
 
     Fallback "Diffuse"
 }

The goal is to make the shader work with multiple renderers the same way did with a single one i.e. have the outline applied on the whole model not on each individual piece. I'm not sure how to. I tried comparing IN.worldPos.y to _Fill but didn't work, I figured if I go with world space and not the texture coord things should improve, no dice.

Any ideas?

Thanks!

Edit: Forums crosslink.