Have you tried using the Biconic Zernike surface type? It's like the Biconic, but with the addition of being able to define some asphere terms along X and Y (as well as defining some Zernike polynomial terms). The sag equation for the Biconic Zernike is as follows:
This can allow you to create some anamorphic surfaces which differ only in their asphere coefficients along X and Y, like below:
For your reference, I've also attached this sample file. Please let us know how this suits you!