How to prevent Unity from modifying tangent

How do I prevent Unity from modifying the tangent vector before it gets to the vertex shader on x360? #pragma glsl_no_auto_normalization has no effect, and I see no other such #pragma for hlsl/360. I'm using the tangent vector for custom data. My custom data should appear as large whole number values in the tangent vector when it gets to the vertex shader, but instead the values are 0 to 1.

Actually tangent.xyz is 0 to 1, but tangent.w has been left alone. So of the 4 values I have stuffed into tangent, 1 is correct and the other 3 are garbage.

What is happening and how can I make it stop?