GrabPass and Transparent geometry

I'm attempting to implement a distortion shader using GrabPass, but am currently using the "Complex Invisible" example shader given on Unity's documentation. Here's the code, for reference:

 Shader "ComplexInvisible" {
     SubShader {
         // Draw ourselves after all opaque geometry
         Tags { "Queue" = "Transparent+10" }
 
         // Grab the screen behind the object into _GrabTexture, using default values
         GrabPass { }
 
         // Render the object with the texture generated above.
         Pass {
             SetTexture [_GrabTexture] { combine texture }
         }
     }
 } 

I am using 2d Toolkit to render my sprites. These shaders render in the transparent queue. For some reason, my GrabPass shader is rendering completely blank. If I switch the shader on my sprites to "Diffuse" instead of the Tk2D shader, then the grabpass works. Unity's Transparent/Diffuse doesn't work, though.

How can I get GrabPass to render transparent objects? I've tried changing the RenderQueue--it seems ineffective.