I'm not even sure this is possible or whether this should be posted here or in the suggestions forum, but I'm being optimistic that it is possible and I'm just missin g something. 
Is it possible to have an "external link" menu item that will either direct to different URLs on desktop vs mobile, or a way to display such a menu item only on one of those?

CSS would work for this. I’m mobile at the moment but it’s doable.