The built-in N AOV is in world space. So how to get a tangent-space N AOV? Like this:
In brief, I read the N AOV, transform the normals from world to tangent space, and then map the normal values to the range 0.5, 1 (I used the range node here, but I could have used Add and Multiply to do the same thing).
Here’s the shader tree. You can save this in a .ass file and import it into Maya or whatever application you use. Then set up an AOV shader to write the custom Ntangent AOV
tangent 1 0 0
normal 0 1 0