actaully jwatte, you can, because there is a mode where it willl calculate a z value as a hemispherical projection of the x and y - giving you a unit length normal.
you must use a signed HILO format, though, and i think it only works for the dotproduct texture shader operations.
a normal is 2dimensional… it has latitude and longitude.
and yes, teh hiloformat uses 2 components for the normalmap, it calculates z = sqrt(1 - xx - yy) in the textureshader and uses it for a textureshader dotproduct. on a normalmap, all normals are facing upwards, => z always greater zero