Creating a shader that overlaps a diffuse with a TexGen ObjectLinear cutout

I was wondering how I would superimpose the red blood from this picture onto the soldier UV from this picture:

with

Here is the code I thought would work, but Alpha values don't seem to factor into shaderlab, which makes sense because otherwise transparency would be easy. The problem is I don't know how to combine anything without +/-/*, so I can never get the shader to look correct. I'm not worried about shader efficiency right now, just methodology.

Without further adeu, here's my code:

 Shader "A/Blood Thermometer"
     {
     Properties
         {
         //_Health ("Health",Range(0,1)) = 1?
         //_InvHealth ("Inverse Health",Range(0,1)) = 0?
         //_Color ("Color", Color) = (1,0,0,1)? //red?
         _MainTex ("Base (RGB)", 2D) = "" //the optimized UV texture for the character
         _BloodTex ("Base (RGBA)", 2D) = "" {TexGen ObjectLinear} //the blood texture, which is placed in world space vertically
         //_BloodTex is the output from a Substance (.SBSAR filetype) made in Substance Designer 2. 
         }
     SubShader
         {
         Pass
             {
             SetTexture [_MainTex]
             SetTexture [_BloodTex] {combine texture} //+ or * previous} doesn't work too well with any variation of _BloodTex color combinations
             }
         }
     }

I'd like to see lighting turned on for the _MainTex, if that's easy. It seems like UsePass would work for the Diffuse, but not the Cutout-shader.

[EDIT:] I found this but I don't know how it helps.

 /*! 
  * \file
  * \author Stig Olavsen <stig.olavsen@randomrnd.com>
  * \author http://www.RandomRnD.com
  * \date © 2011-August-05
  * \brief The shader definition for the HealthBar/HealthBar Transparent shader
  * \details A transparent shader with two texture inputs and a ratio control
  * that lets you set the display ratio of the two textures
  */
 
 Shader "HealthBar/HealthBar Transparent" {
     Properties {
         _MainColor ("Base Color", Color) = (1,0,0,1)
         _BackgroundColor ("Background Color", Color) = (0,0,0,0)
         _MainTex ("Base (RGB)", 2D) = "white" {}
         _BackgroundTex ("Base (RGB)", 2D) = "white" {}
         _Health ("Health", Range(0.0, 1.0)) = 1.0
     }
     SubShader {
         Tags { "Queue"="Transparent" "IgnoreProjector"="True" "RenderType"="Transparent" }
         LOD 200
         
         CGPROGRAM
         #pragma surface surf Lambert alpha
 
         sampler2D _MainTex;
         sampler2D _BackgroundTex;
         fixed4 _MainColor;
         fixed4 _BackgroundColor;
         float _Health;
 
         struct Input 
         {
             float2 uv_MainTex;
         };
 
         void surf (Input IN, inout SurfaceOutput o) 
         {
             half4 c;
             if (IN.uv_MainTex.x < _Health)
             {
                 c = tex2D (_MainTex, IN.uv_MainTex) * _MainColor;
             }
             else
             {
                 c = tex2D (_BackgroundTex, IN.uv_MainTex) * _BackgroundColor;
             }
             o.Albedo = c.rgb;
             o.Alpha = c.a;
         }
         
         ENDCG
     } 
     FallBack "Diffuse"
 }