Axis oriented BRDF

Hi everyone, so I'm working with a custom BRDF shader, and since the shader is for an iOS game, I'm hoping to be as militant as possible about pre-empting potential performance bottlenecks.

Right now, it's just using a directional light to determine the orientation of the diffuse map, but I hope to use the map's effect for all the scene geometry. Now, I'm aware that basically noone "should" use dynamic lighting for ALL geometry for targeting iOS, however I'm hoping theres a workaround here...

Is there a way to simply set the shader to align the diffuse ramp to an axis, and avoid true lighting? I'm exclusively using BRDF maps and vertex colors for all the geometry in this game, so memory footprint taken up by textures is nil...perhaps using the brdf is doable for the scenes?