Shader with arbitrary number of passes

Hi everyone. I am very very new to shader scripting, so please bear with me. I have a model that needs, at runtime, to have textures added to it.

Think of a human torso wearing a t-shirt. The user has buttons in the interface to change the shirt... for instance, it may be different colors, and/or a logo may be added and/or the sleeves may be different colors, etc.

I have found that I can write a shader that has multiple passes. The 1st is just the plain white t-shirt texture. Then the later passes are additive overlays to add in the other parts (which I have as 32 bit PNG files). This is the shader so far:

Code (csharp):

1.  
2. Shader "Test Shader" {
3.     Properties {
4.         _Color ("Main Color", Color) = (1,1,1,1)
5.         _MainTex ("Base (RGB)", 2D) = "white" {}
6.         _OverlayTex1 ("Overlay1 (RGBA)", 2D) = "black" {}
7.         _OverlayTex2 ("Overlay2 (RGBA)", 2D) = "black" {}
8.         _BumpMap ("Bump (RGB) Illumin (A)", 2D) = "bump" {}
9.     }
10.    
11.     SubShader
12.     {
13.         Pass
14.         {
15.             Blend Off
16.             SetTexture [_MainTex] { combine texture }
17.         }
18.            
19.         Pass
20.         {
21.             Blend SrcAlpha OneMinusSrcAlpha
22.             SetTexture [_OverlayTex1] { combine texture }
23.         }  
24.  
25.         Pass
26.         {
27.             Blend SrcAlpha OneMinusSrcAlpha
28.             SetTexture [_OverlayTex2] { combine texture }
29.         }      
30.     }
31.     FallBack "Diffuse", 1
32. }
33.  

That is all fine as long as I want only 2 levels of overlays and I want to supply the blend textures in the editor.

What I want is to define which overlays are added at runtime. There may be more than 2. There may be less. The number of overlays is arbitrary, and will be defined by the user's choices. There is no functional maximum to the number of overlays that will be needed (my actual program is much more advanced than the t-shirt example)

I don't want to make a shader with 100 overlay passes and swap out the ones I need at runtime.

Is there a way to add a repeat loop into the shader that cycles through a list of overlays and only adds the ones I need? Or am I going about this completely wrong?

Thanks for any help!