So that was great advice, and I've gotten this working really well for my purposes now. I added a couple lines of code to an answer posted by user "Owen Reynolds" on this question so that I could manually tweak the scale of the world-aligned repeating texture from the GUI side.

Here's the full code of the working shader, hope it helps somebody else:

Shader "custom_shaders/Transparent_Diffuse_World_Aligned" {

Properties { _Color ("Main Color", Color) = (1,1,1,1) _MainTex ("Base (RGB) Trans (A)", 2D) = "white" {} _Scale ("Texture Scale Multiplier", Float) = 0.1 }

SubShader { Tags {"Queue"="Transparent-101" "IgnoreProjector"="True" "RenderType"="Transparent"} LOD 200 Cull Off Fog {Mode Off}

CGPROGRAM
#pragma surface surf Lambert alpha

sampler2D _MainTex;
float4 _Color;
float _Scale;

struct Input {
    float2 uv_MainTex; // unused
    float3 worldNormal;
    float3 worldPos;
};

void surf (Input IN, inout SurfaceOutput o) {
  // Guess correct planar map from normal. 0.5 is an arbitrary cutoff
  float2 UV;
  // NOTE: assuming no bottom-facing, otherwise use abs()
  if(IN.worldNormal.y>0.5) UV = IN.worldPos.xz; // top
  else if(abs(IN.worldNormal.x)>0.5) UV = IN.worldPos.yz; // side
  else UV = IN.worldPos.xy; // front

  // 0.1 is an arbitrary x10 texture size scale 
  half4 c = tex2D (_MainTex, UV* _Scale)* _Color;
  o.Albedo = c.rgb;
  o.Alpha = c.a;
}

ENDCG

}

Fallback "Transparent/VertexLit" }