Blend effect based on fullscreen texture?

texture foobar < source = "some_image.png";
{
  Width = 3840;
  Height = 2160;
};

sampler foobar_sampler
{
  Texture = foobar;
};

...

float4 blablub(float2 texcoord : TEXCOORD) : SV_TARGET
{
  float4 color = tex2D(ReShade::BackBuffer);
  float4 color_post_effect = ...;

  // Assuming the 0-255 blending value is stored in the red channel of the image.
  // This looks it up and linearizes it to 0.0-1.0 float, since colors are represented as 0.0-1.0 in shaders.
  float blending = tex2D(foobar_sampler, texcoord).r;
  // Linearly interpolate between original color and whatever the post effect did, using that blending value.
  color = lerp(color, color_post_effect, blending);

  return color;
}