So from what I've seen in the API Docs (http://iosapi.xamarin.com/?link=T:Xamarin.Forms.MenuItem/*) Toolbar items don't seem to have an IsVisible property.
I was able to accomplish this by defining a Binding for the "Icon" property and settings it to null when I don't want the toolbar item to display. So far, it has worked well.
Posting here so that anyone else looking to toggle visibility of toolbar items can try it out.