This would be great... but does the game even have staff? I mean, I see scientists and stuff walking around... but aren't those just aesthetic things like all the other guest types?
In any case, a simple and effective way of doing this would be to emulate Zoo Tycoon 2 and make placeable gates that only staff can pass through... of course, we'd need new fencing options too, because using exhibit fences specifically to keep guests in and out would be dumb...