in my opinion it is better to model it at least in a basic form in case you have closeUps or something like that. be aware that zBrush doesn´t create geometry for you. a normal map fakes topology which is good for any detail. but see it more like a texture (which it is in reality) thus it doesn´t behave like a animated geometry. if you have in mind to use displacement you would have to model the outer eyeLid anyway in a basic form to get the right effect. but as soon as you need a working geometry don´t rely on a fake cause that will be noticed by the audience except for the head will never be so close to see it.
my advice with animation ready stuff is -> (concerning hyperrealism) work as much on the geometry and let zBrush be the cherry on the cake. you would need just some two or three extra edgeLoops around the eye to get a working corner of it, that´s all. you don´t have to overexhaust yourself with it. but there´s allways a very noticable difference between real geometry and the effect of normalMapping when it comes to animation.
cheer´s!
everything starts and ends in the right place at the right time.