I don't think we can make masks work together: currently it's pretty much by design that you can only use one mask per object. Masks work by returning an opaque image with the effect "baked in". Most mask effects also don't use the color of the image, just the alpha. So a mask followed by a mask naturally has the effect of using an opaque image on the last mask.
Workaround: use a non-mask version of the shader. E.g. mask HSL, then ordinary hexagonal pixellate. This processes the "baked in" image from the mask, rather than masking the baked-in image with the background, and seems to achieve what you were after. If that's not what you're after, I think you might need to get in to writing some GLSL yourself.