Keep in mind that Grey Foxes climb trees - a fact that is probably unknown to the average guest. Take advantage of this and provide lots of opportunities for this behavior. Also grey foxes are mostly nocturnal so plan on them sleeping during the day - either have a den viewing or plan for a day of night exhibit (though it sounds like you are doing this outdoors).
I would suggest presenting several species around your central fox exhibit - turning the exhibit into an specific ecosystem exhibit instead of a fox exhibit - thus if the foxes are sleeping you still have many other things to see that tell a larger story. You may even engage your guest to be a "Fox Researcher" looking for grey foxes. Show signs of fox activity - scat, tracks, digging, claw marks on trees - Present the story that you are looking for foxes but you may not see them. Make a connection to social media - guest that successfully see the fox can upload pictures to the zoo's facebook.
I think there are a lot of ways to present grey foxes without resorting to the usual "here's the fox pen..."